arm64: sysreg: add Set/Way sys encodings

Cache maintenance ops fall in the SYS instruction class, and KVM needs
to handle them. So as to keep all SYS encodings in one place, this
patch adds them to sysreg.h.

The encodings were taken from ARM DDI 0487A.k_iss10775, Table C5-2.

To make it clear that these are instructions rather than registers, and
to allow us to change the way these are handled in future, a new
sys_insn() alias for sys_reg() is added and used for these new
definitions.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will.deacon@arm.com>