Rework POW conformance to account for subnormals

The conformance rule is identical to the previous one except the error
bound is now calculated based on the minimum normal value in the case
out_ref is subnormal.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: Id2c6a2dba545bc1dc6a3a06aa7264cc2fef57e5d