Add the uint16_t data type

An unsigned 16-bit integer data type for use with image networks.
Limited to only operating with the RESCALE operator for conversion
to signed int16.

Zero point can be 0 or 32768 in the RESCALE to allow for no loss of
precision (by subtracting 32768), or keeping all values as positive,
(zero point=0) with scaling/clipping as defined in the other RESCALE
arguments.

Change-Id: Id1aebab68fa207f8f8cc235fc3fa5d050307198e
Signed-off-by: Eric Kunze <eric.kunze@arm.com>