ARM: imx6q_logic: Enable Pin muxing in SPL

ARM: imx6q_logic: Enable Pin muxing in SPL

With the 256KB of OCRAM available to SPL now, there should be
enough room to enable the pinmuxing in SPL from the device tree.

This patch enables SPL_PINCTRL et al and adds the serial and
usdhc pin mux references to the -u-boot.dtsi file so the pins can be
configured from the device tree.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>