ARM: multiplatform: always pick one CPU type

With the new default platform code, we can always boot using DT
without requiring a board file, but we cannot build a kernel
unless we select at least one CPU core, which breaks some
"randconfig" builds.

This adapts the ARCH_MULTI_V4T and ARCH_MULTI_V5 options so we
always default to a common CPU core if no platform was enabled
that picks something else. The default we pick for ARMv4T is
ARM920T, while for ARMv5 we pick ARM926T.

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