arm64: KVM: Basic ESR_EL2 helpers and vcpu register access

Implements helpers for dealing with the EL2 syndrome register as
well as accessing the vcpu registers.

Reviewed-by: Christopher Covington <cov@codeaurora.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>