board/rdfremont: enable BL1->BL2 based bootflow

Add ROM base and size defines which is required for the BL1 image. Since
RD-Fremont platform doesn't have a dedicated ROM, the ROM space is
carved out in the trusted SRAM and the pre-boot firmware (stage before
AP boot) is expected to configure the trusted SRAM such that the read
access are blocked.

Add required dts file that is required for BL1 and BL2 image load
stages. Also, remove the references of RESET_TO_BL31 in the rdfremont
makefile since BL31 based bootflow is not supported.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>