plat/arm: Add assert for the valid address of dtb information

plat/arm: Add assert for the valid address of dtb information

Added assert in the code to check valid address of dtb information
structure retrieved from fw_config device tree.
This patch fixes coverity defect:360213.

Also, removed conditional calling of "fconf_populate" as "fconf_populate"
function already checks the validity of the device tree address received
and go to panic in case of address is NULL.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: Ib83e4e84a95e2456a12c7a2bb3fe70461d882cba