ipq806x: fix TP-Link Archer VR2600v bootlooping

ipq806x: fix TP-Link Archer VR2600v bootlooping

Timo Schroeder reported:
"The TP-Link Archer VR2600v is stuck in a boot loop on written
snapshot image. It's able to boot using the snapshot uimage
though, but there ath10k firmware can't be found.
21.02.2 release version doesn't have either problem."

The VR2600v has a 512 byte header at the beginning of the
firmware that needs to be accounted for.

Fixes: f6a01d7f5c3d ("ipq806x: convert TP-Link Archer VR2600v to denx,uimage")
Reported-by: Timo Schroeder <der.timosch@gmail.com>
References: <https://github.com/openwrt/openwrt/issues/9467>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>