ARM: dts: stm32: Add DHCOR based Testbench board

Add DT 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. Furthermore, the PMIC Buck3
is always configured from PMIC NVM to cater for both 1V8 and 3V3 SoM
variant.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>