arm64: dts: freescale: imx93-var-som: Add support for ADS7846 touchscreen

arm64: dts: freescale: imx93-var-som: Add support for ADS7846 touchscreen

The VAR-SOM-MX93 integrates an ADS7846 resistive touchscreen controller.
The controller is physically located on the SOM, and its signals are
routed to the SOM pins, allowing carrier boards to make use of it.

This patch adds the ADS7846 node and the appropriate SPI controller.

Signed-off-by: Stefano Radaelli <stefano.radaelli21@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>