soc: add microchip polarfire soc system controller

This driver provides an interface for other drivers to access the
functions of the system controller on the Microchip PolarFire SoC.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220217101349.2374873-2-conor.dooley@microchip.com