ARM: SMP: consolidate the common parts of smp_prepare_cpus()

There is a certain amount of smp_prepare_cpus() which doesn't belong
in the platform support code - that is, code which is invariant to the
SMP implementation.  Move this code into arch/arm/kernel/smp.c, and
add a platform_ prefix to the original function.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>