Don't use zero_normalmem in bl2_platform_setup

zero_normalmem only can zero memory when caches are enabled
and the target memory is a normal memory, otherwise an abort is
generated. In the case of EL3_PAYLOAD_BASE bl2_platform_setup was
calling zero_normalmem with device memory and it generated an abort.

Change-Id: If013603f209a12af488a9c54481f97a8f395b26a
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>