ramips: add support for DLINK DWR-921-C1

The DWR-921-C1 Wireless Routers with LTE embedded modem is based on the
MT7620N SoC.

Specification:

  * MediaTek MT7620N (580 Mhz)
  * 64 MB of RAM
  * 16 MB of FLASH
  * 802.11bgn radio
  * 5x 10/100 Mbps Ethernet (1 WAN and 4 LAN)
  * 2x external, detachable (LTE) antennas
  * UART header on PCB (57600 8n1)
  * 6x LED (GPIO-controlled)
  * 1x bi-color Signal Strength LED (GPIO-controlled)
  * 2x button
  * JBOOT bootloader

The status led has been assigned to the dwr-921-c1:green:sigstrength (lte
signal strength) led. At the end of the boot it is switched off and is
available for lte operation. Work correctly also during sysupgrade
operation.

Installation:
Apply factory image via d-link http web-gui.

How to revert to OEM firmware:
1.) Push the reset button and turn on the power. Wait until LED start
    blinking (~10sec.)
2.) Upload original factory image via JBOOT http (IP: 192.168.123.254)
3.) If http doesn't work, it can be done with curl command:
    curl -F FN=@XXXXX.bin http://192.168.123.254/upg
    where XXXXX.bin is name of firmware file.

Signed-off-by: Giuseppe Lippolis <giu.lippolis@gmail.com>

target/linux/ramips/base-files/etc/hotplug.d/firmware/10-rt2x00-eeprom

@@ -54,7 +54,8 @@ board=$(board_name)
 case "$FIRMWARE" in
 "soc_wmac.eeprom")
 	case $board in
-	dlink,dwr-116-a1)
+	dlink,dwr-116-a1|\
+	dlink,dwr-921-c1)
 		wan_mac=$(jboot_config_read -m -i $(find_mtd_part "config") -o 0xE000)
 		wifi_mac=$(macaddr_add "$wan_mac" 1)
 		jboot_eeprom_extract "config" 0xE000