board: toradex: apalis/colibri imx6: Detect new v1.2 SoM variant

board: toradex: apalis/colibri imx6: Detect new v1.2 SoM variant

Apalis/Colibri iMX6 V1.2 will replace the STMPE811 ADC/Touch controller
which is EOL by the TLA2024 ADC and AD7879 touch controller.

To support this new version, we detect the presence of the TLA2024
during boot and set a new environment variable named "variant". This
will allow us and users to select the correct DT easily.

By probing via I2C we have a robust detection method instead of relying
on the existing "board_rev" environment variable which is set by the
config block. Users can use "variant" in their DT selection and do not
have to map the board revision to a device tree.

"variant" environment variable behaviour:
* Empty or absent for all versions below v1.2 (STMPE811)
* "-v1.2" for all versions starting from v1.2 (TLA2024 + AD7879)

Usage example:
setenv fdtfile imx6q-apalis${variant}-${fdt_board}.dtb

Signed-off-by: Ernest Van Hoecke <ernest.vanhoecke@toradex.com>
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>