perf/arm_dmc620: Add cpumask

Add a cpumask for the DMC620 PMU. As it is an uncore PMU, perf
userspace tool only needs to open a single counter on the CPU
specified by the CPU mask for each event on a given DMC620 device.

Signed-off-by: Xin Yang <xin.yang@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20230613013423.2078397-1-xin.yang@arm.com


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