In Linux, CONFIG_ARCH_OMAP2PLUS is used for OMAP2 or later SoCs.
Rename CONFIG_ARCH_OMAP2 to CONFIG_ARCH_OMAP2PLUS to follow this
naming.

Move the OMAP2+ board/SoC choice down to mach-omap2/Kconfig to slim
down the arch/arm/Kconfig level.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NTom Rini <trini@konsulko.com>