testing/device-samsung-j4lte: New device

Successfully built and booted postmarketOS on Samsung Galaxy J4. It's based on Exynos 7570, same SoC used by samsung-xcover4lte [1].

Currently, these are tested working:

• Boots from internal storage
• Display (console framebuffer is NOT working, kernel panics)
• Framebuffer (additionally with msm-fb-refresher to refresh the screen)
• Touchscreen
• USB Networking (RNDIS)
• Wi-Fi

Tested UI environments:

• XFCE 4
• Console (Kernel log doesn't show up because CONFIG_FRAMEBUFFER_CONSOLE is not enabled, but enabling it panics the kernel. This is a known issue for some Samsung devices, notably samsung-crownlte [2].)

[1] https://wiki.postmarketos.org/wiki/Samsung_Galaxy_Xcover_4_(samsung-xcover4lte)

[2] https://wiki.postmarketos.org/wiki/Samsung_Galaxy_Note_9_(Exynos)_(samsung-crownlte)#Current_status