Add service locator strategy for SP to SP discovery

Using the service locator framework, a new service locator
strategy is added that is suitable for use from within an
SP to discover other service endpoints reachable via FFA.
The required destination SP may be hardcoded in the client
SP or passed in as a configuration parameter.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: If8cd3cd60e43b0f150c5f850211152fec0139f07
7 files changed