MAINTAINERS: Add entry for Microsemi MIPS SoCs

Add myself as a maintainer for the Microsemi MIPS SoCs.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Allan Nielsen <Allan.Nielsen@microsemi.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18857/


Signed-off-by: James Hogan <jhogan@kernel.org>