net: dsa: qca8k: add support for sgmii falling edge

Add support for this in the qca8k driver. Also add support for SGMII
rx/tx clock falling edge. This is only present for pad0, pad5 and
pad6 have these bit reserved from Documentation. Add a comment that this
is hardcoded to PAD0 as qca8327/28/34/37 have an unique sgmii line and
setting falling in port0 applies to both configuration with sgmii used
for port0 or port6.

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>