net: mdio: mscc-miim: convert to a regmap implementation

Utilize regmap instead of __iomem to perform indirect mdio access. This
will allow for custom regmaps to be used by way of the mscc_miim_setup
function.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>