net: mdio: ipq8064: add regmap config to disable REGCACHE

mdio drivers should not use REGCHACHE. Also disable locking since it's
handled by the mdio users and regmap is always accessed atomically.

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>