[mlir][linalg] Define a depthwise 2-D convolution op

This commit defines linalg.depthwise_conv_2d_nhwc for depthwise
2-D convolution with NHWC input/output data format.

This op right now only support channel multiplier == 1, which is
the most common case.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D94966