Use the proper size for tb_fw_cfg_dtb
Use the proper size for tb_fw_cfg_dtb

Currently tb_fw_cfg_dtb size is fixed to max, which is generally a page
(but depend on the platform). Instead, read the actual size of the dtb
with the libfdt "fdt_totalsize" function.
This avoid flushing extra memory after updating the dtb with mbedtls
heap information when shared heap is used.

Change-Id: Ibec727661116429f486464a0c9f15e9760d7afe2
Signed-off-by: Louis Mayencourt <053c3ea6b10c921896b13cd162dd1cf31fffc07a@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware