drivers/fsi: Add documentation for GPIO bindings

Add fsi master gpio device tree binding documentation.

Includes changes from Jeremy Kerr <jk@ozlabs.org>.

Signed-off-by: Christopher Bostic <cbostic@linux.vnet.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>