linux-yocto: drop 5.2 recipes

linux-yocto: drop 5.2 recipes

linux-yocto 5.4 will serve as the versioned reference kernel in
the upcoming release, and -dev will serve as the "newer" kernel.

As such, we drop v5.2 from master, but will continue to update
and support it in released branches.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>