This patch adds sysreg definitions for system registers which are part
of the performance monitors extension. Subsequent patches will make use
of these definitions.

The set of registers is described in ARM DDI 0487A.k_iss10775, Table
D5-9. The encodings were taken from Table C5-6 in the same document.
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>