dt-bindings: fsi: Document the IBM I2C Responder virtual FSI master

The I2C Responder translates I2C commands to CFAM or SCOM operations,
effectively implementing an FSI master.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230612195657.245125-13-eajames@linux.ibm.com


Signed-off-by: Joel Stanley <joel@jms.id.au>