Add SCMI support for Juno platform

This patch adds the memory map region for the SCMI payload memory
and maps the Juno core indices to SCMI power domains via the
`plat_css_core_pos_to_scmi_dmn_id_map` array.

Change-Id: I0d2bb2a719ff5b6a9d8e22e91e1625ab14453665
Signed-off-by: Soby Mathew <soby.mathew@arm.com>