dt-bindings: arm: stm32: Add compatible string for DH electronics DHCOR Testbench board

Add DT compatible string for DHCOR Testbench board, which is a testbench
for testing of DHCOR SoM during manufacturing. This is effectively a
trimmed down version of AV96 board with CSI-2 bridge, HDMI bridge, WiFi,
Audio and LEDs removed and used as GPIOs instead.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>