KVM: arm64: gic-v5: Cache host IRS ID registers

KVM needs access to the host IRS configuration frame to configure
virtual-machine resources. The frame ID registers describe the
capabilities available to KVM.

Cache IRS_IDR2, IRS_IDR3, and IRS_IDR4 rather than extracting their
individual fields into kvm_vgic_global_state. Accessor helpers derive
the required capabilities from the cached registers.

Use the cached IRS_IDR4 VPE capacity to limit the maximum number of
vCPUs, while retaining VGIC_V5_MAX_CPUS as KVM's own upper bound.

Keeping the raw register values lets KVM adjust the exposed
capabilities when needed, as is done for ICH_VTR_EL2, and avoids tying
future EL2 code to the layout of kvm_vgic_global_state.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Link: https://patch.msgid.link/20260904113404.4051341-8-sascha.bischoff@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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