Verify (and fix) the context_id argument passed to the mpentry () by PSCI.

Some older PSCI implementations corrupt (or do not pass) the context_id
argument to newly started secondary cores. Although the ideal solution to this
problem is u-boot update, we can find the correct value for the argument (cpuid)
by comparing of real core mpidr register with the value stored in pcu->mpidr.

MFC after: 2 weeks