riscv: dts: starfive: Add PMU controller node

Add the pmu controller node for the StarFive JH7110 SoC. The PMU needs
to be used by other modules, e.g. VPU,ISP,etc.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Hal Feng <hal.feng@starfivetech.com>
Signed-off-by: Walker Chen <walker.chen@starfivetech.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>