[CMSIS-NN] Pad fusion with QNN Conv2D

[asparkhi]

Pass that fuses nn.pad and qnn.conv2d for CMSIS-NN target.

cc @Mousius @areusch @grant-arm @lhutton1 @manupa-arm

[lhutton1]

Thanks @ashutosh-arm, @SebastianBoblest!