dt-bindings: net: dsa: qca8k: Add SGMII clock phase properties

Add names and descriptions of additional PORT0_PAD_CTRL properties.
qca,sgmii-(rx|tx)clk-falling-edge are for setting the respective clock
phase to failling edge.

Co-developed-by: Matthew Hagan <mnhagan88@gmail.com>
Signed-off-by: Matthew Hagan <mnhagan88@gmail.com>
Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>