MAINTAINERS: add gpio driver to PolarFire entry

Lewis' original GPIO driver patchset didn't add MAINTAINERS coverage of
the driver and I forgot to add it to the existing entry. Make up for the
minimal amount of lost time.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20241107-chewable-spoiler-3e7dd58d3e8d@spud
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Author: ConchuOD

MAINTAINERS

@@ -19837,6 +19837,7 @@ F:	arch/riscv/boot/dts/microchip/
 F:	drivers/char/hw_random/mpfs-rng.c
 F:	drivers/clk/microchip/clk-mpfs*.c
 F:	drivers/firmware/microchip/mpfs-auto-update.c
+F:	drivers/gpio/gpio-mpfs.c
 F:	drivers/i2c/busses/i2c-microchip-corei2c.c
 F:	drivers/mailbox/mailbox-mpfs.c
 F:	drivers/pci/controller/plda/pcie-microchip-host.c