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>