SPM: Refine 'load' interfaces

- Partitions need load info assemling MACROs, move them out of
  'partition_static_load.h'.
- Rename 'partition_static_load.h' into 'spm_load_api.h', and
  create a static load implementation 'static_load.c'.
- Refine the load logic, call API provided in 'spm_load_api.h'.
- Service lookup now list based.
- Rename 'load info' variable in partition and service runtime defs.

Change-Id: I73901094458ff1f11674100f8660eaa44a457d09
Signed-off-by: Ken Liu <Ken.Liu@arm.com>