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

MAINTAINERS

@@ -17142,6 +17142,7 @@ L:	linux-riscv@lists.infradead.org
 S:	Supported
 F:	arch/riscv/boot/dts/microchip/
 F:	drivers/mailbox/mailbox-mpfs.c
+F:	drivers/rtc/rtc-mpfs.c
 F:	drivers/soc/microchip/
 F:	include/soc/microchip/mpfs.h