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 <louis.mayencourt@arm.com>