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.