MAINTAINERS: add spi to PolarFire SoC entry

Add the newly introduced spi driver to the existing PolarFire SoC entry.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220607073833.2331539-3-conor.dooley@microchip.comSigned-off-by: Mark Brown <broonie@kernel.org>

MAINTAINERS

@@ -17143,6 +17143,7 @@ S:	Supported
 F:	arch/riscv/boot/dts/microchip/
 F:	drivers/mailbox/mailbox-mpfs.c
 F:	drivers/soc/microchip/
+F:	drivers/spi/spi-microchip-core.c
 F:	include/soc/microchip/mpfs.h
 
 RNBD BLOCK DRIVERS