Fix FF-A prototype to run StMM on top of Hafnium

Minor fixes to accomodate running StMM SEL0 SP on top of Hafnium (built
with VHE support):
-FFA_SUCCESS 32 func id.
-Mem perm set macro to check 32b ffa success returned func id.
-FDT compatible string for manifest passed to StMM SP.
-Prevent adjusting permissions for zero sized sections.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>