Allow bias tensors to be broadcast

Bias tensors often have the same value for
each channel. To allow for this the number of
bias channels, BC, is permitted to be 1 or the
output channel size. If BC == 1 then the
bias is broadcast.

Change-Id: I3262a6c699bd045f44201e2a123f9b792b9f67c9
Signed-off-by: Dominic Symes <dominic.symes@arm.com>