Update POW conformance to match LOG changes

Following changes to the conformance of LOG which relaxed requirements
with inputs near 1.0, POW is updated to match the implied conformance of
EXP(MUL(y, LOG(x))).

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I5bcdc875c5ce56fb185f4ba6e8ccb1632c358e0a