mdio-gpio: Add reset functionality to mdio-gpio driver(v2).

This patch adds phy reset functionality to mdio-gpio driver. Now
mdio_gpio_platform_data has new member as function pointer which can be
filled at the bsp level for a callback from phy infrastructure. Also the
mdio-bitbang driver fills-in the reset function of mii_bus structure.

Without this patch the bsp level code has to takecare of the reseting
PHY's on the bus, which become bit hacky for every bsp and
phy-infrastructure is ignored aswell.

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