arm64: dts: ls1088a: add missing PMU node

The Performance Manager Unit was not described in the DTS
which meant performance event monitoring was not possible.

This was exposed by a change to the PMU handling in KVM
in 5.11-rc3 which now prevents a PMU being exposed to a
guest when the host does not provide one:
"KVM: arm64: Don't access PMCR_EL0 when no PMU is available"

Signed-off-by: Mathew McBride <matt@traverse.com.au>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>