ipq40xx: update to 5.15.15

Update the IPQ40xx kernel to 5.15.15.

This is a huge kernel update for IPQ40xx, it finally converts the target
to use DSA.

Now we finally have switch ports showing up as proper network devices.
It brings most of the DSA available features like FDB offloading, VLAN
filtering with offloading and just proper regular VLAN-s.
MDB offloading will follow soon.

Kernel bridge VLAN filtering and IGM snooping had to be enabled along
with IP multicast.

Switch to using defconfig instead of the whole config to avoid unecessary
refreshes.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>

Author: robimarko