arm64: dts: morello: Add CMN PMU

Although CMN-Skeena is mildly modified for the Morello hardware
architecture, it still identifies itself as CMN-600 r3p1. Since
there are also no documented changes to its PMU functionality,
we can make the PMU accessible via the standard CMN-600 binding.
In general, PMU registers are non-functional on CMN Fast Models,
so this is only meaningful for the real SDP hardware.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>