rtc: Add driver for Microchip PolarFire SoC

Add support for the built-in RTC on Microchip PolarFire SoC

Co-Developed-by: Daire McNamara <daire.mcnamara@microchip.com>
Signed-off-by: Daire McNamara <daire.mcnamara@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220601123320.2861043-2-conor.dooley@microchip.com