Interface: Interface provided to NS side

These files provide a reference interface mplementation for integration
with OS running on the NS side. This has been tested to work with
RTX scheduler.
Modifications may be required while integrating other OS.

Change-Id: I4845584465c5df0bc574de31564a0789154c0dd5
Signed-off-by: Ashutosh Singh <ashutosh.singh@arm.com>
Co-Authored-By: Marc Moreno Berengue <marc.morenoberengue@arm.com>
Co-Authored-By: Antonio de Angelis <antonio.deangelis@arm.com>