ARM: omap2+: rename config to ARCH_OMAP2PLUS and consolidate Kconfig

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: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/sniper_defconfig b/configs/sniper_defconfig
index ca2762a..c041392 100644
--- a/configs/sniper_defconfig
+++ b/configs/sniper_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 # CONFIG_SYS_THUMB_BUILD is not set
+CONFIG_ARCH_OMAP2PLUS=y
 CONFIG_OMAP34XX=y
 CONFIG_TARGET_SNIPER=y
 # CONFIG_SPL_NAND_SUPPORT is not set