libpldm: Add parent to pldm_entity_node structure

Add its own parent entity to the pldm_entity_node structure, and then
use pldm_entity_get_parent to get the entity's parent.

Also, add UTest code for it.

Tested: Built pldm daemon successfully and UTest passes.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I067f82ad6d03f3cffd2ca24cd771274777901b61