apm821xx: disable WNDR4700 5.4 image

apm821xx: disable WNDR4700 5.4 image

The compressed image that the buildbots are building is too large for
the netgear uboot and it crashes and soft-bricks the device.

| Uncompressing Kernel Image ...
| LZMA: uncompress or overwrite error 1 - must RESET board to recover

The whole target likely needs to be switched zImage which is a major
hassle due to powerpc's legacy bootwrapper setup as compared to ARM.
So for now, disable the device.

Reported-by: Wiktor Stasiak (FS#3258)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>