Add DSP support for fully connected int16

Authors: Sebastian-Larsson, oscarandersson8218 (GitHub)

Adding DSP implementation for fully connected int16 as well a unit
test.

Also replacing SXTB(__ROR()) by __SXTB16_RORn in read_and_pad, since it
is used by this and it is faster for GCC.

Change-Id: I4a40a3f9137435e822957c6c8f971e65c1bf9706