powerpc/fsl_msi: Handle msi-available-ranges better

Now handles multiple ranges, doesn't make assumptions about interrupt
specifier format, and doesn't claim interrupts that don't correspond to an
available range.

Also has some better error checking.

The device tree binding is updated to clarify some existing assumptions.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>