ARM: dts: sun8i: R40: Add PMU node

The ARM Cortex-A7 cores used in the Allwinner R40 SoC have their usual
Performance Monitoring Unit (PMU), which allows perf to use hardware
events.
The SoC integrator just needs to connect each per-core interrupt line
to the GIC. The R40 manual does not really mention those IRQ lines, but
experimentation in U-Boot shows that interrupts 152-155 are connected to
the four cores (similar to the A20).

Tested on a Bananapi M2 Berry, with perf and taskset to confirm the
association between cores and interrupts.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>