dt-bindings: arm: stm32: Add compatible string for DH electronics STM32MP13xx DHCOR DHSBC board

Add DT compatible string for DH electronics STM32MP13xx DHCOR SoM and
DHSBC carrier board. This stm32mp135f-dhcor-dhsbc board is a stack of
DHCOR SoM based on STM32MP135F SoC (900MHz / crypto capabilities)
populated on DHSBC carrier board.

The SoM contains the following peripherals:
- STPMIC (power delivery)
- 512MB DDR3L memory
- eMMC and SDIO WiFi module

The DHSBC carrier board contains the following peripherals:
- Two RGMII Ethernet ports
- USB-A Host port, USB-C peripheral port, USB-C power supply plug
- Expansion connector

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>