configs/stm32f746_disco_sd: new defconfig

configs/stm32f746_disco_sd: new defconfig

The commit adds support for STM32F746G-DISCO board. The discovery kit is
a complete demonstration and development platform for STMicroelectronics
Arm Cortex-M7-core-based STM32F746NG.

Board support package includes the following components:
- mainline Linux kernel 5.15.165
- mainline U-Boot 2024.07
- default packages from buildroot

Link: https://www.st.com/en/evaluation-tools/32f746gdiscovery.html
Co-Developed-by: Waldemar Brodkorb <[email protected]>
Signed-off-by: Waldemar Brodkorb <[email protected]>
Signed-off-by: Dario Binacchi <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>