imx: kontron-sl-mx8mm: Prepare for I2C display detection in environment script

Enable the I2C bus and set a env variable for the reset GPIO of the touch
controller. This allows us to probe the panel in a script.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>