arm: dts: imx7: add pinctrl defines

Add pinctrl defines for NXP i.MX 7Solo/7Dual SoC. The pinctrl format
is compatible to the Linux kernel, hence this file is a simple copy
from the Linux kernel (commit 97f5c1817b7e).

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>