arm_gic: Mask the un-supported priority bits

The GICv2 allows the implementation to implement a variable number
of priority bits; unimplemented bits in the priority registers
are read as zeros, writes ignored. We were previously always
implementing a full 8 bits of priority, which is allowed but not
what the real hardware typically does (which is usually to have
4 or 5 bits of priority).

Add a new device property to allow the number of implemented
property bits to be specified.

Signed-off-by: Sai Pavan Boddu <sai.pavan.boddu@xilinx.com>
Message-id: 1582537164-764-2-git-send-email-sai.pavan.boddu@xilinx.com
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: improved commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>