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 <peng.fan@nxp.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Cc: Stefano Babic <sbabic@denx.de>