fix(el3-spmc): compute full FF-A V1.1 desc size

This patch fixes an issue in spmc_ffa_fill_desc.

In order to compute the spmc_shm_get_v1_1_descriptor_size,
fragment_length which is a fraction of the descriptor size is used as
desc_size parameter. It has to be replaced with the
full V1.0 descriptor size(obj->desc_filled).

Ran a subset of our tests and they are passing.

Change-Id: Ia4bbc5dabf0b77fa53d923ff609ee48ecd5bf549
Signed-off-by: vallau01 <valentin.laurent@trustonic.com>
Signed-off-by: Lukas Hanel <lukas.hanel@trustonic.com>