net: macb: Add support for partial store and forward

When the receive partial store and forward mode is activated, the
receiver will only begin to forward the packet to the external AHB
or AXI slave when enough packet data is stored in the packet buffer.
The amount of packet data required to activate the forwarding process
is programmable via watermark registers which are located at the same
address as the partial store and forward enable bits. Adding support to
read this rx-watermark value from device-tree, to program the watermark
registers and enable partial store and forwarding.

Signed-off-by: Maulik Jodhani <maulik.jodhani@xilinx.com>
Signed-off-by: Pranavi Somisetty <pranavi.somisetty@amd.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>