The usb20_phy clock needs to be registered for the driver to be able
to get and enable a clock. Currently the usb phy clocks are registered
from board files, which will not be called during a device tree based
boot.

To be able to probe correctly usb form a device tree boot, register
the usb phy clocks from the DT specific init.

Unfortunately, davinci does not have proper clock support on device tree
yet, so by registering the clock from the DT specific file we are
forced to hardcode the parent clock, and cannot select refclkin as
parent for any of the phy clocks of the da850 family.

As none of the current da850 based boards currently in mainline use
refclkin as source. I guess we can live with this limitation until clocks
are correctly represented through CCF/device tree.
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
[Added error checking]
Signed-off-by: David Lechner <david@lechnology.com>
[nsekhar@ti.com: typo fixes in commit message]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>