Rename RELATIVE verify mode to MAX_RELATIVE

RELATIVE allows error relative to some fixed 'max' value, i.e. fixed
error for all outputs in an operation.

Renaming it to MAX_RELATIVE allows us to add a second mode,
OUTPUT_RELATIVE, which will allow error relative to each output in an
operator.

The OUTPUT_RELATIVE mode will be coming as a separate patch.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: Ia70549c19ec781b1172cb1a2a7fd0da581489ca5