net: ethernet: ti: davinci_mdio: switch to readl/writel()

Switch to readl/writel() APIs, because this is recommended
API and the MDIO block is reused on Keystone 2 SoCs
where LE/BE modes are supported.

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>