docs(porting-guide): update a reference

The BL31 part has been there forever and the PSCI reference is neither
at section 3.3 or directly below. Update this to locate the section more
easily.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I9a86e4ef13d1ac5da743917493f63ddd7690e087
1 file changed