Avoid FEAT_FP16 requirement unnecessarily

* The conversion between FP32 and FP16 is part of the base
  instruction set and does not require FEAT_FP16.
  The equivalent functions in the test framework need to change
  to avoid the need for FEAT_FP16.

Signed-off-by: Viet-Hoa Do <viet-hoa.do@arm.com>

Approved-by: Felix Johnny Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>
Approved-by: Anton Bondarenko <anton.bondarenko@arm.com>