[ARM] SMP: don't set cpu_*_map in smp_prepare_boot_cpu

The recent addition of boot_cpu_init() implements the initialisation
of the online, present and possible cpu maps for the boot CPU, so
there is no reason to duplicate this in the architecture
smp_prepare_boot_cpu() hook.

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