fsl-layerscape: drop obsolete PPA secure firmware support

PPA was a secure firmware developed in-house which is no longer
supported and replaced by TF-A quite some years ago. Drop support
for it.

Signed-off-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>