linux-yocto-dev: bump to v5.12-rc

The -dev kernel has moved to v5.12, as it will be the latest avialable
in the upcoming release. core-image-kernel-dev sanity has passed for
the supported arches.

So we bump the version to match the active branches.

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