ARM: OMAP2+: add dependencies on ARCH_MULTI_V6/V7

CONFIG_ARCH_OMAP2PLUS depends on (ARCH_MULTI_V6 || ARCH_MULTI_V7) as of
a0694861 "ARM: OMAP2+: Enable ARCH_MULTIPLATFORM support", but the
individual OMAP2/3/4/5 and AM33XX platforms can all be selected independent
of what we are building for, which is a bug and prevents us from easily
building e.g. an ARMv7-only defconfig.

This makes ARCH_OMAP2 depend on ARCH_MULTI_V6 and the others depend on
ARCH_MULTI_V7, to ensure we really only build the platforms for the
CPUs we have enabled in the global multiplatform configuration step.

Cc: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>