bindings: ipmi: Add binding for SSIF BMC driver

Add device tree binding document for the SSIF BMC driver.

Signed-off-by: Quan Nguyen <quan@os.amperecomputing.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Message-Id: <20221004093106.1653317-3-quan@os.amperecomputing.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>