testing/device-samsung-a51: New device (MR 5887)

Successfully built and booted postmarketOS on Samsung Galaxy A51. It's based on Exynos 9611 SoC.

Currently, these are tested working:

- Boots from internal storage
- Display (DECON driver needs to be patched. Included in the kernel package)
- Framebuffer (additionally with msm-fb-refresher to refresh the screen)
- Touchscreen
- USB Networking (RNDIS)
- Wi-Fi

Tested UI environments: XFCE 4 & Console.

Wiki: https://wiki.postmarketos.org/wiki/Samsung_Galaxy_A51_(samsung-a51)