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

docs/user/supported_devices.rst

@@ -6,6 +6,7 @@ ath79-generic
 
 * AVM
 
+  - FRITZ!WLAN Repeater 300E [#avmflash]_
   - Fritz!WLAN Repeater 450E [#avmflash]_
   - Fritz!Box 4020 [#avmflash]_
 

targets/ath79-generic

@@ -22,6 +22,10 @@ device('avm-fritz-box-4020', 'avm_fritz4020', {
 	factory = false,
 })
 
+device('avm-fritz-wlan-repeater-300e', 'avm_fritz300e', {
+	factory = false,
+})
+
 device('avm-fritz-wlan-repeater-450e', 'avm_fritz450e', {
 	factory = false,
 })