Remove usage of half_float

- Replace all instances of half_float::half with ct::float16
- Remove some special code in fpTrunc for handling bfloat16 as cfloat.h
  can do that special handling now
- Update serialization_lib submodule
- Update unit tests to match new serialization_lib version

Signed-off-by: Eirene Vlassi Pandi <eirene.vlassipandi@arm.com>
Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I6374f2712e943a756322940eea753b7c2f57f943