MAINTAINERS: update ARM PMU profiling and debugging for arm64

Will Deacon maintains the profiling and debugging code under both
arch/arm and arch/arm64. Update MAINTAINERS to reflect this, in
preparation for adding myself as a reviewer of said 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>