Mediatek pinctrl drivers call mtk_gpiochip_register() to bind the child
gpio controller as part of mtk_pinctrl_common_probe(). This breaks
gpiohog support because the gpio controller is bound too late for
DM_FLAG_PROBE_AFTER_BIND (set while binding hogs) to work.

Move the mtk_gpiochip_register() to mtk_pinctrl_common_bind() and call
this as the .bind method of each of the mediatek pinctrl drivers.

Signed-off-by: Chris Webb <chris@arachsys.com>