fix(plat/sgi): include dyn_cfg in BL31 sources

This patch includes arm_dyn_cfg.c into BL31 sources. This source has
function to provide Mbed TLS heap information to caller.

Signed-off-by: Sayanta Pattanayak <sayanta.pattanayak@arm.com>
Change-Id: Ib801c3643fe4a38ca7bf64c33f278c01c949375e