arm64/kvm: vgic: use SYS_DESC()

Almost all of the arm64 KVM code uses the sysreg mnemonics for AArch64
register descriptions. Move the last straggler over.

To match what we do for SYS_ICH_AP*R*_EL2, the SYS_ICC_AP*R*_EL1
mnemonics are expanded in <asm/sysreg.h>.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: kvmarm@lists.cs.columbia.edu
Acked-by: Christoffer Dall <cdall@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>