Fix incorrect clamping of Bool

When ofm datatype is Bool, we were incorrectly
clamping output to [0, 255].
As True is represented as -1, it was clamped to 0.

Fixed by having IntegerMin and IntegerMax handle Bool.

Change-Id: I14ad89e128ab3339318f9967f8693959e3378652
Signed-off-by: Björn Davidsson <bjoern.davidsson@arm.com>