powerpc/powermac: Constrain 'cpu_sleep()' for AIM to mpc745x

Rename cpu_sleep() to mpc745x_sleep() to denote what it's actually
intended for. This function is very G4-specific, and will not work on
any other CPU. This will afterward eliminate a
platform_smp_timebase_sync() call by directly updating the timebase
instead.

(cherry picked from commit b6d8f3b517dec010f3dfad1b33e9945eaa606be5)