pinctrl: cygnus: add initial IOMUX driver support

This adds the initial driver support for the Broadcom Cygnus IOMUX
controller. The Cygnus IOMUX controller supports group based mux
configuration but allows certain pins to be muxed to GPIO individually

Signed-off-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Tested-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>