KVM: arm64: add SYS_DESC()

This patch adds a macro enabling us to initialise sys_reg_desc
structures based on common sysreg encoding definitions in
<asm/sysreg.h>. Subsequent patches will use this to simplify the KVM
code.

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