yocto/kas: Pin Poky to align linux and linux-rt versions

Cassini pinned Poky to an old version, which makes the linux version
6.6.50. While we need 6.6.54 for the fix of a network issue.

Now pin the Poky version in sw-ref-stack to a newer one, align the
version of linux and linux-rt to 6.6.54.

The pin should be removed when Cassini publish new release with
equal or higher kernel version.

Signed-off-by: Michael Zhao <michael.zhao2@arm.com>