bmips: add support for SmartRG SR505n

bmips: add support for SmartRG SR505n

Specifications:
- SoC: Broadcom BCM63168 dual 400MHz MIPS
- Flash: 16MB SPI NOR W25Q128WFG
- RAM: 128MB DDR3 W631GG6KB-15
- Ethernet: 1x 1000M, 3x 100M
- Wifi: BCM435F
- 1x USB 2.0 port
- 3x Button
- 12x LED

Flashing via serial
- Connect to the 3.3V TTL UART on the board
  (J6 pinout Vcc Rx Tx Gnd) at 115200-8-N-1
- Press any key in the serial console when powering up the board to enter
  the CFE prompt
- Configure an interface on your workstation to static IP 192.168.1.100
  and connect it to the board
- Start a TFTP server with the firmware image
- On the CFE prompt, enter the command
  "f 192.168.1.100:openwrt-bmips-bcm63268-smartrg_sr505n-squashfs-cfe.bin"

Signed-off-by: Kyle Hendry <kylehendrydev@gmail.com>
[Remove unneeded LED labels]
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>