MLBEDSW-8789 Handle depthwise convolution with depth multiplier > 1

Set depth multiplier in GraphBuilder.
Add depth multiplier decomposition in DecomposeDepthwiseConv2D.

Change-Id: Ic1408dadba6e9587bff725308d17b5778d184be4
Signed-off-by: Fredrik Svedberg <fredrik.svedberg@arm.com>