MAINTAINERS: Add a maintainer for MSCC MIPS SoCs

Microsemi has been bought by Microchip and Microchip is supporting those
SoCs.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Woojung Huh <Woojung.Huh@microchip.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-kernel@vger.kernel.org