arm64/perf: Access pmu register using <read/write>_sys_reg

changed pmu register access to make use of <read/write>_sys_reg
from sysreg.h instead of accessing them directly.

Signed-off-by: Ashok Kumar <ashoks@broadcom.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>