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>