testing/device-samsung-j4lte: New device (MR 5936)

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