ar71xx: add support for compex wpj558

OpenWrt can be flashed with following uboot commands:

tftpboot 0x80500000 openwrt-ar71xx-generic-wpj558-16M-squashfs-sysupgrade.bin
erase 0x9f030000 +$filesize
cp.b $fileaddr 0x9f030000 $filesize

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44620 3c298f89-4303-0410-b956-a3cf2f4a3e73