net: dsa: qca8k: add explicit SGMII PLL enable

Support enabling PLL on the SGMII CPU port. Some device require this
special configuration or no traffic is transmitted and the switch
doesn't work at all. A dedicated binding is added to the CPU node
port to apply the correct reg on mac config.
Fail to correctly configure sgmii with qca8327 switch and warn if pll is
used on qca8337 with a revision greater than 1.

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