ARM: OMAP2+: Enable ARCH_MULTIPLATFORM support

Flip on multiplatform support for omap2+.

No changes to omap2plus_defconfig needed, but please note
that you may need to update your custom config files to
make sure you have:

CONFIG_ARCH_MULTIPLATFORM=y
CONFIG_ARCH_MULTI_V7=y
CONFIG_ARCH_OMAP2PLUS=y

And may need CONFIG_ARCH_MULTI_V6=y if booting omap2 boards.

Cc: Russell King <linux@arm.linux.org.uk>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>