qoriq: switch to kernel 5.15
The testing kernel has been running fine for months on a rather complex
network setup, with one exception being a crash in
__xfrm_state_delete(). This crash has been fixed in kernel 5.15.82, so
let's switch the qoriq target to kernel 5.15 by default.
Signed-off-by: Stijn Tintel <[email protected]>