dmaengine: dw-axi-dmac: Honour snps,block-size

The snps,block-size DT property declares the maximum block size for each
channel of the dw-axi-dmac. However, the driver ignores these when
setting max_seg_size and uses MAX_BLOCK_SIZE (4096) instead.

To take advantage of the efficiencies of larger blocks, calculate the
minimum block size across all channels and use that instead.

See: https://github.com/raspberrypi/linux/issues/6256

Signed-off-by: Phil Elwell <phil@raspberrypi.com>