MAINTAINERS: Use wildcard pattern for ARM PMU headers

Looks like arm_pmuv3.h isn't caught by the ARM PMU maintainers entry.
Fix it with a wildcard.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20230915202421.2706446-1-oliver.upton@linux.dev


Signed-off-by: Will Deacon <will@kernel.org>