Add missing REQUIREs for OpPow

- Add REQUIRE statements for OpPow for range checking.
- Remove test set 2 with negative base inputs.
- Remove FP special test data.
- Refactor a to x, b to y, c to z to match with the specification.

Signed-off-by: Jerry Ge <jerry.ge@arm.com>
Change-Id: Ib89a2dba7b030f196af083c7160fe5ce28393fcf