MAINTAINERS: update riscv/microchip entry

Update the RISC-V/Microchip entry by adding the microchip dts
directory and myself as maintainer

Reviewed-by: Lewis Hanly <lewis.hanly@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>