build: juno: scp_romfw: Update build to support DT build flow

Update juno scp_romfw firmware variant to support using DT build flow.

Adjust CMakeLists and Firmware.cmake to select device tree base source file
and DT supported modules for the variant.

Add in Juno scp_romfw DT specific config overrides where required.

Signed-off-by: Mike Leach <mike.leach@linaro.org>