dt-bindings: input: Add documentation for IBM Operation Panel

Document the bindings for the IBM Operation Panel, which provides
a simple interface to control a server. It has a display and three
buttons.
Also update MAINTAINERS for the new file.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20220809204147.238132-2-eajames@linux.ibm.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>