MAINTAINERS: add documentation file for Microchip EIC section

Add documentation file to Microchip EIC section.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230523070637.224476-3-claudiu.beznea@microchip.com


Signed-off-by: Rob Herring <robh@kernel.org>