docs(spm): memory region nodes definition

Update the documentation related with memory region nodes
of SP's FF-A manifest, to relate to changes from patches [1].

[1] https://review.trustedfirmware.org/q/topic:%22ja%252Fmem_region_fix%22+(status:open%20OR%20status:merged)

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I16595ec581b0ad9d2c20fca8dab64b6fd9ad001a
1 file changed