MAINTAINERS: add auto-update driver to mpfs entry

Rob's scripts were broken by the lack of a maintainer for this file,
while trying to fix an integration issue in linux-next.
Add it to the existing entry for PolarFire SoC drivers so that when the
next bug is found the contributor knows where to send it.

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>