SPM: FVP: Introduce port of SPM

SPM: FVP: Introduce port of SPM

This initial port of the Secure Partitions Manager to FVP supports BL31
in both SRAM and Trusted DRAM.

A document with instructions to build the SPM has been added.

Change-Id: I4ea83ff0a659be77f2cd72eaf2302cdf8ba98b32
Co-authored-by: Douglas Raillard <douglas.raillard@arm.com>
Co-authored-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Co-authored-by: Achin Gupta <achin.gupta@arm.com>
Co-authored-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>