omap: switch default kernel to 5.15

omap: switch default kernel to 5.15

Switch to kernel 5.15 although it is marked as source only. It is marked
as source only because the eth0 interface can't be added to the br-lan
default interface resulting in a broken ethernet [0]. There is a
workaround to set a higher pvd (e.g. to 4000) in the dts [1].

Other functionality was already tested on a Beaglebone Black for
+6month now without issues.

Merging this is important to finally branch a new release. We want to
get rid completely of 5.10 kernel. Without this commit we get following
error: "Missing kernel version/hash file for 5.10." Hopfully someone
will fix the board in the rc phase.

- [0] https://github.com/openwrt/openwrt/issues/11953
- [1] https://github.com/openwrt/openwrt/issues/11953#issuecomment-1419078990

Signed-off-by: Alexey Smirnov <s.alexey@gmail.com>
[rewrite commit message]
Signed-off-by: Nick Hainke <vincent@systemli.org>