MLBEDSW-2082: Add Exp support

- Added int8 and int16 Exp support, implemented as LUT.
- Added generic 8bit and 16bit LUT table functions following
the implementation in the latest reference. If new ops are added
by the reference, they can easily be implemented in Vela using
the generic functions.
- Moved convert_to_lut to lut.py to have all LUT related code in
one file.
- Updated SUPPORTED_OPS.md

Change-Id: I388e76ea4b39162313599a5341cfb9bad71a782c
Signed-off-by: Johan Alfven <johan.alfven@arm.com>