MAINTAINERS: add musb to PolarFire SoC entry

Add the newly introduced musb driver to the existing PolarFire SoC entry.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220613114642.1615292-3-conor.dooley@microchip.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>