dt-bindings: arm: stm32: Add compatible string for Linux Automation LXA TAC

Add DT compatible string for Linux Automation GmbH Test Automation
Controllers (LXA TAC).
LXA TACs are a development tool for embedded devices with a focus on
embedded Linux devices.

As of now there are two STM32MP157 based hardware generations (Gen 1 and
Gen 2) that have most of their hardware config in common.
In the future there will also be a STM32MP153 based hardware generation.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>