Documentation: Rename gpio controller name from cygnus to iproc

Renamed gpio controller's driver name from cygnus to iproc to make it
more generic so that all iProc based SoCs having the same gpio controller
could use this.

Signed-off-by: Pramod Kumar <pramodku@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>