Add the Performance Monitoring Unit (PMU) device tree node to the H6
.dtsi, which tells DT users which interrupts are triggered by PMU
overflow events on each core. The numbers come from the manual and have
been checked in U-Boot and with perf in Linux.

Tested with perf record and taskset on a Pine H64.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>