usb: phy: mxs: Add DT bindings to configure TX settings

The TX settings can be calibrated for particular hardware.  The
phy is reset by Linux, so this cannot be handled by the bootloader.

The TRM mentions that the maximum resistance should be used for the
DN/DP calibration in order to pass USB certification.

The values for the TX registers are poorly described in the TRM.
The meanings of the register values were taken from another
NXP-provided document:
https://community.nxp.com/message/566147#comment-566912



Acked-by: Peter Chen <peter.chen@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jaret Cantu <jaret.cantu@timesys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>