powerpc: add support for MPIC message register API

Some MPIC implementations contain one or more blocks of message registers
that are used to send messages between cores via IPIs.  A simple API has
been added to access (get/put, read, write, etc ...) these message registers.
The available message registers are initially discovered via nodes in the
device tree.  A separate commit contains a binding for the message register
nodes.

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