dt-bindings: net: dsa: qca8k: Document qca,led-open-drain binding

Document new binding qca,ignore-power-on-sel used to ignore
power on strapping and use sw regs instead.
Document qca,led-open.drain to set led to open drain mode, the
qca,ignore-power-on-sel is mandatory with this enabled or an error will
be reported.

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