USB: ehci-tegra: add probing through device tree

Rely on platform_data being passed through auxdata for now; more elaborate
bindings for phy config and tunings to be added.

v2: moved vbus-gpio check to the helper function, added check for !of_node,
    added usb2 clock to board-dt table.

Signed-off-by: Olof Johansson <olof@lixom.net>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Acked-by: Grant Likely <grant.likely@secretlab.ca>