powerpc: document the FSL MPIC message register binding

This binding documents how the message register blocks found in some FSL
MPIC implementations shall be represented in a device tree.

Signed-off-by: Meador Inge <meador_inge@mentor.com>
Signed-off-by: Jia Hongtao <B38951@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>