pinctrl: imx: Use function callbacks for SCU related functions

Use function callbacks for SCU related functions in pinctrl-imx.c
in order to support the scenario of PINCTRL_IMX is built in while
PINCTRL_IMX_SCU is built as module, all drivers using SCU pinctrl
driver need to initialize the SCU related function callback, and
no need to check CONFIG_PINCTRL_IMX_SCU anymore, hence stub functions
also can be removed.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Link: https://lore.kernel.org/r/1599552721-24872-1-git-send-email-Anson.Huang@nxp.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>