MAINTAINERS: add microchip soc binding directory to microchip soc driver entry

I noticed that there's technically not an explicit maintainer for this
directory, even if the files currently in it are covered by either the
Mircochip FPGA or AT91 entries. Add it to the entry covering the
corresponding driver directory.

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>