dt-bindings: mailbox: miv-ihc: remove redundant quotes causing warnings

commit 6e3eaf3786d223dab10492a217f75d42b3d108d3 from
https://github.com/linux4microchip/linux.git

 linux-6.6-mchp+fpga

Fix:
../Documentation/devicetree/bindings/mailbox/microchip,miv-ihc.yaml:5:10: [error] string value is redundantly quoted with any quotes (quoted-strings)

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: harish h <harish.h@windriver.com>