ARM: dts: imx6ull: add additional pinfunc defines for i.MX 6ULL

On i.MX 6ULL, the pin MUX and CTRL register of BOOT_MODEx and TAMPERx
pins are available through IOMUXC_SNVS. Add additional pinfunc defines.

Signed-off-by: Bai Ping <ping.bai@nxp.com>
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>