soc: starfive: Add StarFive JH71XX pmu driver

Add pmu driver for the StarFive JH71XX SoC.

As the power domains provider, the Power Management Unit (PMU) is
designed for including multiple PM domains that can be used for power
gating of selected IP blocks for power saving by reduced leakage
current. It accepts software encourage command to switch the power mode
of SoC.

Signed-off-by: Walker Chen <walker.chen@starfivetech.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>