pinctrl: fsl: add scu based pinctrl support

Some i.MX SoCs (e.g. MX8QXP and MX8QM) contain a system controller
that is responsible for controlling the pad setting of the IPs that
are present. Communication between the host processor running an OS
and the system controller happens through a SCU protocol.

This patch classifies the pad settings into two categories: MMIO and SCU.
For the original MMIO method, no functional changes except organize them
into a few imx_*_mmio() functions. Besides that, we add the SCU based
Pad Mux and Pinconf setting support which are implemented in pinctrl-scu.c.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>