hw/intc/arm_gicv3_kvm.c: Stop using GIC_MIN_BPR constant

hw/intc/arm_gicv3_kvm.c: Stop using GIC_MIN_BPR constant

The GIC_MIN_BPR constant defines the minimum BPR value that the TCG
emulated GICv3 supports.  We're currently using this also as the
value we reset the KVM GICv3 ICC_BPR registers to, but this is only
right by accident.

We want to make the emulated GICv3 use a configurable number of
priority bits, which means that GIC_MIN_BPR will no longer be a
constant.  Replace the uses in the KVM reset code with literal 0,
plus a constant explaining why this is reasonable.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220512151457.3899052-4-peter.maydell@linaro.org
Message-id: 20220506162129.2896966-3-peter.maydell@linaro.org