ARM: PSCI: export common PSCI function declarations for C code

Some common PSCI functions are written in assembly, but it should be
possible to use them from C code.

Add function declarations for C code to consume.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>