SST: Use asset ID instead of handle in all APIs

This patch updates the SST interfaces to:
- use asset ID instead of asset handle as the object is always
  close at the end of each request
- change the asset ID type from uint16_t to uint32_t to allow
  longer asset IDs which can contain vendor specific ID codes
- remove tfm_sst_get_handle API
- updates all files which depend on these APIs and
  their documentation

Change-Id: I4a844e548cc7cdfe2277b007717003a0263e45d1
Signed-off-by: Marc Moreno <marc.morenoberengue@arm.com>
33 files changed