ARM: All OMAP2PLUS machines use omap2 directory so just add one entry

All OMAP2PLUS arch based machines makes use of mach-omap2 directory.
So just add one entry so that there is no need to patch this file
for any OMAP2+ devices.
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>

arch/arm/Makefile

@@ -164,9 +164,7 @@ machine-$(CONFIG_ARCH_MXS)		:= mxs
 machine-$(CONFIG_ARCH_NETX)		:= netx
 machine-$(CONFIG_ARCH_NOMADIK)		:= nomadik
 machine-$(CONFIG_ARCH_OMAP1)		:= omap1
-machine-$(CONFIG_ARCH_OMAP2)		:= omap2
-machine-$(CONFIG_ARCH_OMAP3)		:= omap2
-machine-$(CONFIG_ARCH_OMAP4)		:= omap2
+machine-$(CONFIG_ARCH_OMAP2PLUS)	:= omap2
 machine-$(CONFIG_ARCH_ORION5X)		:= orion5x
 machine-$(CONFIG_ARCH_PICOXCELL)	:= picoxcell
 machine-$(CONFIG_ARCH_PNX4008)		:= pnx4008