ARM: dts: imx6ull: add IOMUXC SNVS instance

The i.MX 6ULL features another IOMUX Controller called IOMUXC
SNVS which allows to control BOOT_MODE and TAMPER pins. Add the
controller to the i.MX 6ULL specific imx6ull.dtsi device tree.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>