bcm63xx: add support for the Sercomm H500-s

bcm63xx: add support for the Sercomm H500-s

Sercomm H500-s is an xDSL dual band wireless router based on Broadcom
BCM63167 SoC.

Hardware:
   SoC:          Broadcom BCM63167
   CPU:          BMIPS4350 V8.0, 400 MHz, 2 cores
   Flash:        NAND 128 MiB
   RAM:          DDR3 128 MiB
   Ethernet:     4x 10/100/1000 Mbps
   Switch:       BCM53134S
   Wireless:     802.11b/g/n: BCM435f (integrated)
                 802.11ac:    Quantenna QT3740BC (onboard SoC)
   USB:          1x 2.0
   LEDs/Buttons: 11x / 2x

Flash instruction, web UI:
  1. Reset to defaults using the reset button if the admin password is
     unknown
  2. Login into the web UI as admin.
     Address:  http://192.168.0.1
     User:     admin
     Password: VF-ESVodafone-H-500-s or l033i-h500s
  3. Go to Settings -> Firmware Update, and select the Openwrt factory
     firmware
  4. Update the firmware.
  5. Wait until it finish, the device will reboot with Openwrt installed
     on the alternative image partitions keeping the stock firmware in
     the former.

Notes:
  - The patch also adds support for the lowi version. Only the factory
    firmware is different.
  - The integrated Wifi in the Broadcom Soc isn't still supported.
  - The Quantenna 802.11ac wifi works ok, but needs to be configured with
    the Quantenna client application. It can't be configured with Luci
    nor any iw command since it's a separated subsystem linked via
    ethernet.
  - The BCM53134S external switch is managed via MDIO which isn't
    supported in this target. Therefore it will behave as a dumb switch.

Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>