net: ethernet: ti: davinci_cpdma: add weight function for channels

The weight of a channel is needed to split descriptors between
channels. The weight can depend on maximum rate of channels, maximum
rate of an interface or other reasons. The channel weight is in
percentage and is independent for rx and tx channels.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>