bmips: bcm6358: add support for Huawei HG556a ver C

bmips: bcm6358: add support for Huawei HG556a ver C

Specifications:
- SoC: Broadcom BCM6358 dual 300MHz MIPS
- Flash: 16MB NOR Macronix MX29GL128EHT2I-90G
- RAM: 64MB DDR
- Ethernet: 4x 100M
- Wifi: Ralink RT3062F
- 3x USB 2.0 port
- 4x Button
- 13x LED
- RJ11 2x FXS VoIP (unsupported)
- RJ11 xDSL (unsupported)

Install instructions:
- Assign static IP 192.168.1.100 to PC.
- Unplug the power source.
- Press the RESTART button at the router, don't release it yet!
- Plug the power source and wait at least 15 seconds.
- Release the RESTART button.
- Browse to http://192.168.1.1 with your PC.
- Upload the openwrt-bmips-bcm6358-huawei_hg556a-c-squashfs-cfe.bin file.
- Wait some minutes until the firmware upgrade completes.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>