ARM: OMAP: Fix Kconfig warning for omap1

Commit 21278aea ("ARM: use menuconfig for sub-arch menus") improved
the sub-arch menus, but accidentally caused new warnings for omap1.
This was because the commit added a menu entry around config ARCH_OMAP
bool entry where the menu had depends on ARCH_MULTI_V6 || ARCH_MULTI_V7.

As ARCH_OMAP is shared between omap1 and omap2plus, let's fix the
issue by defining ARCH_OMAP in the shared plat-omap/Kconfig.

Fixes: 21278aea ("ARM: use menuconfig for sub-arch menus")
Reported-by: Andreas Ruprecht <rupran@einserver.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>