powerpc/mpic: Add ability to reset a core via MPIC

We need the ability to reset cores for use with kexec/kdump for
SMP systems. Calling this function with the specific core you want
to reset will cause the CPU to spin in reset.

Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>