KVM: arm/arm64: Extract GICv3 max APRn index calculation

KVM: arm/arm64: Extract GICv3 max APRn index calculation

As we are about to access the APRs from the GICv2 uaccess interface,
make this logic generally available.

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>
(cherry picked from commit 50f5bd5718df9e71d1f4ba69a6480dbad54b2f24)

CVE-2017-5753
CVE-2017-5715
CVE-2017-5754

Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>