MAINTAINERS: add PolarFire SoC's RTC

Add an entry for PolarFire SoC's RTC drver to the existing support
for PolarFire SoC.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220601123320.2861043-3-conor.dooley@microchip.com