kirkwood: add support for Cisco ON100

kirkwood: add support for Cisco ON100

The Cisco ON100 device is a Kirkwood based router:

    SoC: Marvell 88F6282 1600Mhz
    SDRAM memory: 512MB DDR3 1333Mhz
    Gigabit ethernet: 2x Marvell 88E1310 (over RGMII)
    Flash memory: 512MB
    2 bi-colour status LEDs (green/red)
    1 Reset button
    1 USB 2.0 port (on back)
    1 SDIO slot (on back)

This commit adds a target profile of "Cisco Systems ON100" under the target
system "Marvell Kirkwood".

Flashing can be performed over tftp, once "dhcp" has been issued:
 tftpboot ${loadaddr} lede-kirkwood-on100-squashfs-factory.bin
 nand erase 0x0c0000 ${filesize}
 nand write ${loadaddr} 0x0c0000 ${filesize}

Once flashed, set environment variables to boot:
 setenv bootcmd nand read \${loadaddr} 0x0c0000 0x540000\; setenv bootargs
  \; bootm
 saveenv

Signed-off-by: Makoto Takeuchi <mak0@lxsys.co.uk>