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>

Author: ConchuOD

MAINTAINERS

@@ -18641,6 +18641,7 @@ F:	Documentation/devicetree/bindings/usb/microchip,mpfs-musb.yaml
 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/i2c/busses/i2c-microchip-corei2c.c
 F:	drivers/mailbox/mailbox-mpfs.c
 F:	drivers/pci/controller/pcie-microchip-host.c