pinctrl: imx6ul: add IOMUXC SNVS pinctrl driver for i.MX 6ULL

On i.MX 6ULL, the BOOT_MODEx and TAMPERx pin MUX and CTRL registers
are available in a separate IOMUXC_SNVS module. Add support for the
IOMUXC_SNVS module to the i.MX 6UL pinctrl driver.

Signed-off-by: Bai Ping <ping.bai@nxp.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>