doc: Update BL1 and BL2 boot flow ...

Updated the document for BL1 and BL2 boot flow to capture
below changes made in FCONF

1. Loading of fw_config and tb_fw_config images by BL1.
2. Population of fw_config and tb_fw_config by BL2.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: Ifea5c61d520ff1de834c279ce1759b53448303ba

doc: Add binding document for fconf. ...

Complete the documentation with information on how to write a DTS for
fconf. This patch adds the bindings information for dynamic
configuration properties.

Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>
Change-Id: Ic6d9f927df53bb87315c23ec5a8943d0c3258d45