ARM: keystone: Replace platform SMP with PSCI

The KS2 boot monitor supports PSCI commands. These are already defined
in DT for KS2. We can drop this platform override and remove related
code without changing SMP. Do this here.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20221026145033.17905-1-afd@ti.com