dt-bindings: fsi: Document gpios

The FSI master has some associated GPIOs that may be present on some
hardware configurations.

Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20200728025527.174503-2-joel@jms.id.au


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