ARM: Kconfig: move platform selection into its own Kconfig file

Mostly just for better organization for now. This matches what is done on
some other platforms including ARM64. This also lets us start to reduce
the number of mach- directories that only exist to store the platform
selection.

Start with "Platform selection" and ARCH_VIRT.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>