dt-bindings: net: snps,dwmac: per channel irq

Add dt-bindings for per channel irq.

Signed-off-by: Rohan G Thomas <rohan.g.thomas@intel.com>
Signed-off-by: Swee Leong Ching <leong.ching.swee@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>