hw/intc/arm_gicv3: Use correct number of priority bits for the CPU

Make the GICv3 set its number of bits of physical priority from the
implementation-specific value provided in the CPU state struct, in
the same way we already do for virtual priority bits.  Because this
would be a migration compatibility break, we provide a property
force-8-bit-prio which is enabled for 7.0 and earlier versioned board
models to retain the legacy "always use 8 bits" behaviour.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220512151457.3899052-6-peter.maydell@linaro.org
Message-id: 20220506162129.2896966-5-peter.maydell@linaro.org