fwk: Define module count at pre-process time

This commit removes the manual module counting logic in the framework,
which occurs at module initialization time, in favour of declaring the
module count ahead of time in the module index header.

This saves on runtime initialization time, and enables further
optimizations within `fwk_module.c`.

Change-Id: I54c33a8eaba22bc5e293ab9b0d2bcbdc7f04f3d3
Signed-off-by: Chris Kay <chris.kay@arm.com>