pinctrl: Add pinctrl driver for i.MX8

pinctrl: Add pinctrl driver for i.MX8

Add pinctrl driver for i.MX8. The pads configuration is controlled
by SCU, so need to ask SCU to configure pads through scfw API.
Add pinctrl-scu to invoke sc_pad_set to configure pads.
Add a new flag IMX8_USE_SCU to differentiate i.MX8 from other platforms
which could directly configure pads from Acore side.
Add CONFIG_PINCTRL_IMX8 as the built gate.

Signed-off-by: Peng Fan <[email protected]>
Reviewed-by: Anatolij Gustschin <[email protected]>
Cc: Stefano Babic <[email protected]>