kas: Pin meta-st-stm32mp to avoid breaking build with latest regs in lockfile

This is a legacy build configuration that needs updating and refactoring. To keep the build working at least, let's pin the layer ref.

The refspec syntax is deprecated. Instead use the new 'branch' and 'commit' keywords. This also allows us to use a lockfile that is parsed by kas automatically for specifying the exact commit hashs.