net: macb: Add support for jumbo frames

Add support for jumbo frames in driver if support in IP is communicated
via DT property. Update the register settings and masks as required.

Add hook to accept mtu size changes for jumbo.

jumbo_max_length register is set to 16K on reset in Cadence IP but
Zynq Ultrascale MPSoC supports only upto 10K. Hence this register will be
written with value passed from devicetree.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>