dsa: qca8k: add mac_power_sel support

Add missing mac power sel support needed for ipq8064/5 SoC that require
1.8v for the internal regulator port instead of the default 1.5v.
If other device needs this, consider adding a dedicated binding to
support this.

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