arm64: sysreg: add physical timer registers

This patch adds sysreg definitions for system registers used to control
the architected physical timer. Subsequent patches will make use of
these definitions.

The encodings were taken from ARM DDI 0487A.k_iss10775, Table C5-6.

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>