ath79: move base-files for ZyXEL NBG6716

Due to rebase of the base-files split patch after adding support
for the ZyXEL NBG6716, its base-files were added to the generic
subtarget, although the device belongs to the nand subtarget.

This moves the definitions to their proper locations.

Fixes: 0130022baec0 ("ath79: split base-files into subtargets")
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
[improve commit message, rebase]
Signed-off-by: David Bauer <mail@david-bauer.net>