powerpc/fsl: Document rapidio node binding-information

This document is created for powerpc rapidio and rmu nodes in dts file.
These nodes can support two rapidio ports and message units. In addition,
It explicates the properties and gives examples about rapidio and rmu nodes.

Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Jin Qing <b24347@freescale.com>
Signed-off-by: Liu Gang <Gang.Liu@freescale.com>
Acked-by: Alexandre Bounine <alexandre.bounine@idt.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>