of/mdio-gpio: Simplify the way device tree support is implemented.

This patch cleans up the way device tree support is added in mdio-gpio
driver. I found lot of code duplication which is not necessary.
Also strangely a new platform driver was also introduced for device tree
support. All this forced me to do this cleanup patch.
After this patch, the driver probe checks the of_node pointer to get the
data from device tree.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>