Add fsi master gpio device tree binding documentation.

Includes changes from Jeremy Kerr <jk@ozlabs.org>.
Signed-off-by: NChristopher Bostic <cbostic@linux.vnet.ibm.com>
Signed-off-by: NJeremy Kerr <jk@ozlabs.org>
Signed-off-by: NJoel Stanley <joel@jms.id.au>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>