dt-bindings: net: dsa: qca8k: Document qca,sgmii-enable-pll

Document qca,sgmii-enable-pll binding used in the CPU nodes to
enable SGMII PLL on MAC config.

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