pinctrl: imx6: support i.MX6ULL

There two iomuxc for i.MX6ULL. one iomuxc is compatible is i.MX6UL,
the other iomuxc is for SVNS usage, similar with the one in mx7.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Simon Glass <sjg@chromium.org>