To make it possible to use the hw_config device tree for dynamic
configuration in BL31 on the Arm Juno platform. A placeholder hw_config
has been added that is included in the FIP and a Juno specific BL31
setup has been added to populate fconf with the hw_config.

Juno's BL2 setup has been updated to align it with the new behavior
implemented in the Arm FVP platform, where fw_config is passed in arg1
to BL31 instead of soc_fw_config. The BL31 setup is expected to use the
fw_config passed in arg1 to find the hw_config.
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>
Change-Id: Ib3570faa6714f92ab8451e8f1e59779dcf19c0b6