powerpc/mpic_msgr: add lock for MPIC message global variable

Also fix issue of accessing invalid msgr pointer issue. The local
msgr pointer in fucntion mpic_msgr_get will be accessed before
getting a valid address which will cause kernel crash.

Signed-off-by: Mingkai Hu <Mingkai.hu@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>