ath79-generic: add AVM FRITZ!WLAN Repeater 300E support (#2406)

Specifications:
* SoC: AR7242 (Virian 400MHz)
* RAM: 64 MB DDR (W9751G6JB-25)
* Flash: 16MB SPI flash (S25FL129PIF)
* WiFi: AR9382 (2.4/5GHz) + 2x SE2595L
* LAN: 1x1000M (PEF7071V)

To install via EVA bootloader, a FTP connection need to be
established to 192.168.178.1 within the first seconds after power on:

  ftp> quote USER adam2
  ftp> quote PASS adam2
  ftp> binary
  ftp> debug
  ftp> passive
  ftp> quote MEDIA FLSH
  ftp> put lede-ar71xx-generic-fritz300e-squashfs-sysupgrade.bin mtd1