powerpc: Document the Open PIC device tree binding

This binding documents several properties that have been in use for quite
some time, and adds one new property 'pic-no-reset', which controls the
runtime initialization behavior of the PIC.  More specifically, the presence
of 'pic-no-reset' mandates that the PIC shall not be reset during runtime
initialization and that any initialization related to interrupt sources
shall be limited to sources explicitly referenced in the device tree.  This
functionality is useful in AMP systems where multiple OSes are sharing the
PIC and the reinitialization of the PIC can interfere with OSes that are
already up and running.

The interrupt specifier definition is based off of Stuart Yoder's FSL MPIC
binding.

Signed-off-by: Meador Inge <meador_inge@mentor.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Stuart Yoder <stuart.yoder@freescale.com>
Cc: Hollis Blanchard <hollis_blanchard@mentor.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>