Revert "mvebu: switch default kernel to 5.15"

Revert "mvebu: switch default kernel to 5.15"

This reverts commit 5429411f732ba76eced30b5b596ec9c0374d0965 as upstream
in commit e5f31552674e ("ethernet: fix PTP_1588_CLOCK dependencies") has
changed `PTP_1588_CLOCK` dependency handling in 5.15 kernel.

That currently leads to `CONFIG_NET_DSA_MV88E6XXX=m` in images produced
by buildbots due to `CONFIG_ALL_KMODS=y` config option being used in
those builds, which leads to a broken LAN bridge network on several
devices.

References: https://lists.infradead.org/pipermail/openwrt-devel/2022-December/039950.html
Signed-off-by: Petr Štetiar <ynezz@true.cz>