dt-bindings: net: macb: Add optional property tsu-clk

Add TSU clock frequency to be used for 1588 support in macb driver.

Signed-off-by: Harini Katakam <harini.katakam@xilinx.com>
Reviewed-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>