ARM: dts: Add fck for cpsw mdio for omap variants

In preparation for dropping legacy platform data and custom ti,hwmods
property, we need to make functional clock available for mdio for the
SoCs so the the mdio driver can find it.

The mdio hardware currently relies on a mdio_hwmod to manage the clock
for omap variants. This is wrong though as there are no separate
sysconfig registers for mdio. All the cpsw related components are just
children of the gmac module.

Note that since mdio is a child of cpsw, just doing pm_runtime_get()
in the mdio driver enables the clock. However, since mdio is also used
by davinci that does not implement runtime PM, let's just add the fck
for now.

Also note that am437x mdio already has a clock, let's update it to
not use the legacy clock naming to unify things further.

Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Cc: Keerthy <j-keerthy@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>