linux-yocto: introduce 5.14 reference kernel

Each release of the kernel is not typically used as a versioned
reference kernel, but since 5.13 has gone EOL upstream and was
never part of a release, we take the opportunity to udpate our
reference to 5.14.

5.13 will be removed in subsequent commits, once all dependent
BSPs and layers have been updated.

No issues were found across all architectures, images in qemu
boot testing.

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