dt-bindings: microchip,eic: Add bindings for the Microchip EIC

Add DT bindings for Microchip External Interrupt Controller.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210927063657.2157676-2-claudiu.beznea@microchip.com