ARM: use menuconfig for sub-arch menus

The System Type menu is getting quite long with platforms and is
inconsistent in handling of sub-arch specific options. Tidy up the menu
by making platform options a menuconfig entry containing any platform
specific config items.

[arnd: change OMAP part according to suggestion from
 Tony Lindgren <tony@atomide.com>]

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>