MAINTAINERS: Add a maintainer for Microsemi switches

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

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Woojung Huh <Woojung.Huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>