ARM: Kconfig: clean up platform selection

The top-level platform selection is mostly meaningless these days after
almost everything is sorted below the CONFIG_ARCH_MULTIPLATFORM, with
the only exception being the 20+ year old StrongARM based machines.

Make this more consistent by removing the entire choice statement and
moving the StrongARM specific options into regular platform specific
Kconfig files.

The three platforms (footbridge, rpc and sa1100) are still mutually
exclusive and cannot coexist with other ARMv4/v5 machines, but since
there are only three of them and we will not add more, this can be
expressed using Kconfig 'depends on' statements.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>