trusty: save/restore FPU registers in world switch

trusty: save/restore FPU registers in world switch

Currently, Trusty OS/LK implemented FPU context switch in internal
thread switch but does not implement the proper mechanism for world
switch. This commit just simply saves/restores FPU registes in world
switch to prevent FPU context from being currupted when Trusty OS uses
VFP in its applications.

It should be noted that the macro *CTX_INCLUDE_FPREGS* must be defined
in trusty.mk if Trusty OS uses VFP

Signed-off-by: Aijun Sun <aijun.sun@spreadtrum.com>