ARM: orion: fix include path

Now that CONFIG_ARCH_MULTIPLATFORM can be disabled anywhere,
there is a build failure for plat-orion:

arch/arm/plat-orion/irq.c:19:10: fatal error: plat/irq.h: No such file or directory

Make the include path unconditional.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm/plat-orion/Makefile

@@ -2,7 +2,7 @@
 #
 # Makefile for the linux kernel.
 #
-ccflags-$(CONFIG_ARCH_MULTIPLATFORM) := -I$(srctree)/$(src)/include
+ccflags-y := -I$(srctree)/$(src)/include
 
 orion-gpio-$(CONFIG_GPIOLIB)      += gpio.o
 obj-$(CONFIG_PLAT_ORION_LEGACY)   += irq.o pcie.o time.o common.o mpp.o