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>

Documentation/devicetree/bindings/arm/stm32/stm32.yaml

@@ -143,7 +143,9 @@ properties:
       - description: Octavo OSD32MP15x System-in-Package based boards
         items:
           - enum:
-              - lxa,stm32mp157c-mc1 # Linux Automation MC-1
+              - lxa,stm32mp157c-mc1      # Linux Automation MC-1
+              - lxa,stm32mp157c-tac-gen1 # Linux Automation TAC (Generation 1)
+              - lxa,stm32mp157c-tac-gen2 # Linux Automation TAC (Generation 2)
           - const: oct,stm32mp15xx-osd32
           - enum:
               - st,stm32mp157