[PATCH] powerpc: Allow for ppc_md restart, power_off, and halt to be NULL

On a number of embedded reference boards there isn't always a
way to reset, power_off, or halt the board.  Rather than having
each board implement a spin loop just let the generic code do
it.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>