ath79: fix first reboot issue on Netgear WNDR4300 v2 and WNDR4500 v3

ath79: fix first reboot issue on Netgear WNDR4300 v2 and WNDR4500 v3

From the Netgear u-boot GPL code[1]. Bootloader always unconditionally
marks block 768, 1020 - 1023 as bad blocks on each boot. This may lead
to conflicts with the OpenWrt nand driver since these blocks may be good
blocks. In this case, U-boot will override the oob of these blocks so
that break the ubi volume. The system will be damaged after first reboot.
To avoid this issue, manually skip these blocks by using "mtd-concat".

[1] https://www.downloads.netgear.com/files/GPL/EX7300v2series-V1.0.0.146_gpl_src.tar.bz2.zip

Fixes: https://github.com/openwrt/openwrt/issues/8878
Tested-by: Yousaf <yousaf465@gmail.com>
Signed-off-by: Shiji Yang <yangshiji66@qq.com>