powerpc: Make MPIC honor the "pic-no-reset" device tree property

This property, defined in the Open PIC binding, tells the kernel not to use
the reset bit in the global configuration register.  Additionally, its
presence mandates that only sources which are actually used (i.e. appear in
the device tree) should have their VECPRI bits initialized.

Although, "pic-no-reset" can be used for the same use cases that
"protected-sources" is covering, the "protected-sources" implementation was
left completely intact.  This is a more pragmatic approach as there are
already several existing systems which use protected sources.  If
"pic-no-reset" *and* "protected-sources" are both used, however, then
"pic-no-reset" takes precedence in terms of the init behavior and the
sanity checks done by protected sources will still take place.

Signed-off-by: Meador Inge <meador_inge@mentor.com>
Cc: Hollis Blanchard <hollis_blanchard@mentor.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>