sh: Fix -Wmissing-include-dirs warnings for various platforms

The 0day bot reports a lot of warnings (or errors due to CONFIG_WERROR)
like this:

  cc1: error: arch/sh/include/mach-hp6xx: No such file or directory [-Werror=missing-include-dirs]

Indeed, arch/sh/include/mach-hp6xx does not exist.

While -Wmissing-include-dirs is only a W=1 warning, it may be
annoying when CONFIG_BTRFS_FS is enabled because fs/btrfs/Makefile
unconditionally adds this warning option.

arch/sh/Makefile defines machdir-y for two purposes:

 - Build platform code in arch/sh/boards/mach-*/
 - Add arch/sh/include/mach-*/ to the header search path

For the latter, some platforms use arch/sh/include/mach-common/
instead of having its own arch/sh/include/mach-*/.

Drop unneeded machdir-y to omit non-existing include directories.

To build arch/sh/boards/mach-*/, use the standard obj-y syntax in
arch/sh/boards/Makefile.
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202302190641.30VVXnPb-lkp@intel.com/Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/20230219141555.2308306-1-masahiroy@kernel.orgSigned-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>

arch/sh/Makefile

@@ -116,30 +116,14 @@ export ld-bfd
 
 # Mach groups
 machdir-$(CONFIG_SOLUTION_ENGINE)		+= mach-se
-machdir-$(CONFIG_SH_HP6XX)			+= mach-hp6xx
 machdir-$(CONFIG_SH_DREAMCAST)			+= mach-dreamcast
 machdir-$(CONFIG_SH_SH03)			+= mach-sh03
-machdir-$(CONFIG_SH_RTS7751R2D)			+= mach-r2d
-machdir-$(CONFIG_SH_HIGHLANDER)			+= mach-highlander
 machdir-$(CONFIG_SH_MIGOR)			+= mach-migor
-machdir-$(CONFIG_SH_AP325RXA)			+= mach-ap325rxa
 machdir-$(CONFIG_SH_KFR2R09)			+= mach-kfr2r09
 machdir-$(CONFIG_SH_ECOVEC)			+= mach-ecovec24
-machdir-$(CONFIG_SH_SDK7780)			+= mach-sdk7780
 machdir-$(CONFIG_SH_SDK7786)			+= mach-sdk7786
 machdir-$(CONFIG_SH_X3PROTO)			+= mach-x3proto
-machdir-$(CONFIG_SH_SH7763RDP)			+= mach-sh7763rdp
-machdir-$(CONFIG_SH_SH4202_MICRODEV)		+= mach-microdev
 machdir-$(CONFIG_SH_LANDISK)			+= mach-landisk
-machdir-$(CONFIG_SH_LBOX_RE2)			+= mach-lboxre2
-machdir-$(CONFIG_SH_RSK)			+= mach-rsk
-
-ifneq ($(machdir-y),)
-core-y	+= $(addprefix arch/sh/boards/, \
-	     $(filter-out ., $(patsubst %,%/,$(machdir-y))))
-endif
-
-# Common machine type headers. Not part of the arch/sh/boards/ hierarchy.
 machdir-y					+= mach-common
 
 # Companion chips

arch/sh/boards/Makefile

@@ -18,3 +18,22 @@ obj-$(CONFIG_SH_APSH4A3A)	+= board-apsh4a3a.o
 obj-$(CONFIG_SH_APSH4AD0A)	+= board-apsh4ad0a.o
 
 obj-$(CONFIG_SH_DEVICE_TREE)	+= of-generic.o
+
+obj-$(CONFIG_SOLUTION_ENGINE)	+= mach-se/
+obj-$(CONFIG_SH_HP6XX)		+= mach-hp6xx/
+obj-$(CONFIG_SH_DREAMCAST)	+= mach-dreamcast/
+obj-$(CONFIG_SH_SH03)		+= mach-sh03/
+obj-$(CONFIG_SH_RTS7751R2D)	+= mach-r2d/
+obj-$(CONFIG_SH_HIGHLANDER)	+= mach-highlander/
+obj-$(CONFIG_SH_MIGOR)		+= mach-migor/
+obj-$(CONFIG_SH_AP325RXA)	+= mach-ap325rxa/
+obj-$(CONFIG_SH_KFR2R09)	+= mach-kfr2r09/
+obj-$(CONFIG_SH_ECOVEC)		+= mach-ecovec24/
+obj-$(CONFIG_SH_SDK7780)	+= mach-sdk7780/
+obj-$(CONFIG_SH_SDK7786)	+= mach-sdk7786/
+obj-$(CONFIG_SH_X3PROTO)	+= mach-x3proto/
+obj-$(CONFIG_SH_SH7763RDP)	+= mach-sh7763rdp/
+obj-$(CONFIG_SH_SH4202_MICRODEV)+= mach-microdev/
+obj-$(CONFIG_SH_LANDISK)	+= mach-landisk/
+obj-$(CONFIG_SH_LBOX_RE2)	+= mach-lboxre2/
+obj-$(CONFIG_SH_RSK)		+= mach-rsk/