KVM: arm64: Use common GICv3 sysreg definitions

Now that we have common definitions for the GICv3 register encodings,
make the KVM code use these, simplifying the sys_reg_descs table.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: kvmarm@lists.cs.columbia.edu