xics: Add support for reduced priority field size

xics: Add support for reduced priority field size

This makes the ICS support less than the 8 architected bits
and sets the soc to use 3 bits by default.

All the supported bits set translates to "masked" (and will read
back at 0xff), any small value is used as-is.

Linux doesn't use priorities above 5, so this is a way to save
silicon. The number of supported priority bits is exposed to the
OS via the config register.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>