SPM: Remove thread management in SFN backend

In SFN backend, there is only one thread, so thread switch
will never happen. This patch is to remove the thread
context in partition struct and thread management in SPM.

Signed-off-by: Sherry Zhang <sherry.zhang2@arm.com>
Change-Id: Ie31dc5271f5fb1b32bbfc5312b02c736a85b9103
12 files changed