KVM: arm64: Move SPSR_EL1 to the system register array

SPSR_EL1 being a VNCR-capable register with ARMv8.4-NV, move it to
the sysregs array and update the accessors.

Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>