This reverts commit 97c77fff28cf001399f33c7bc1ec6687ba18450b as commit
8be6350f6646 ("generic: 5.15: allow MV88E6xxx built-in when PTP support
disabled") contains the fix, so lets enable kernel 5.15 on mvebu again.
Signed-off-by: Petr Štetiar <ynezz@true.cz>