kernel: generic: fix fonfxc uimage parser

kernel: generic: fix fonfxc uimage parser

We cannot distinguish between fonfxc uImage and generic uImage because
fonfxc uImage header is almost same as generic uImage, except padding
length after image name.

The fonfxc uImage parser is available when specifying directly with DT
compatible property.  So this patch adds check if the partition DT node
is compatible with the parser.

Ref: https://bugs.openwrt.org/index.php?do=details&task_id=2413
Fixes: a1c6a316d299 ("ramips: add support for Fon FON2601")
Signed-off-by: NOGUCHI Hiroshi <drvlabo@gmail.com>
[commit light touches and removed C code comment]
Signed-off-by: Petr Štetiar <ynezz@true.cz>