arm64: Add macros to read/write system registers

Rather than crafting custom macros for reading/writing each system
register provide generics accessors, read_sysreg and write_sysreg, for
this purpose.

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