ipq40xx: 5.15: add testing kernel version

ipq40xx: 5.15: add testing kernel version

Add 5.15 kernel as a testing kernel version in the Makefile.

Linksys EA6350v3/EA8300/MR8300 will not build with buildbot settings and
should be disabled when the target is switched, unless the image size is
reduced again.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Robert Marko <robert.marko@sartura.hr>
[add comment for increased kernel size]
Signed-off-by: Sungbo Eo <mans0n@gorani.run>