ti: k3: common: Add PSCI stubs

These functions are used for the PSCI implementation and are needed to
build BL31, but we cannot implement them until we add several more drivers
related to ti-sci so these are only stubs for now.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Benjamin Fair <b-fair@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>