riscv: dts: microchip: add pac1934 power-monitor to icicle

The binding for this landed in v6.9, add the description. In the
off-chance that there were people carrying local patches for this based
on the driver shipped on the Microchip website (or vendor kernel) both
the binding and sysfs filenames changed during upstreaming.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>