plat/arm: Update the fw_config load call and populate it's information

plat/arm: Update the fw_config load call and populate it's information

Modified the code to do below changes:

1. Migrates the Arm platforms to the API changes introduced in the
   previous patches by fixing the fconf_load_config() call.
2. Retrieve dynamically the address of tb_fw_config using fconf
   getter api which is subsequently used to write mbedTLS heap
   address and BL2 hash data in the tb_fw_config DTB.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>
Change-Id: I3c9d9345dcbfb99127c61d5589b4aa1532fbf4be