fvp: trusty: Move dynamic xlat enable to platform

Rather than letting the Trusty makefile set the option to enable dynamic
translation tables, make platforms do it themselves.

This also allows platforms to replace the implementation of the
translation tables library as long as they use the same function
prototypes.

Change-Id: Ia60904f61709ac323addcb57f7a83391d9e21cd0
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>