dt-bindings: net: cdns,macb: Add rx-watermark property

watermark value is the minimum amount of packet data
required to activate the forwarding process. The watermark
implementation and maximum size is dependent on the device
where Cadence MACB/GEM is used.

Signed-off-by: Pranavi Somisetty <pranavi.somisetty@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>