spm-mm: correcting instructions to build SPM for FVP

Out of two possible implementation of Secure Partition Manager(SPM)
currently only Management mode (MM) design is supported and the support
for SPM based on SPCI Alpha 1 prototype has been removed.

Earlier both implementation used common build flag "ENABLE_SPM" but it
has since been decoupled and MM uses a separate build FLAG "SPM_MM".

Instructions to build it for FVP was still using "ENABLE_SPM", which has
beend corrected in this patch.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I505b98173d6051816436aa602ced6dbec4efc776
1 file changed