MAINTAINERS: add myself as arm perf reviewer

As suggested by Will Deacon, add myself as a reviewer of the ARM PMU
profiling and debugging code.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>