drivers: phy: add calibrate method

Some quirky UDCs (like dwc3 on Exynos) need to have their phys calibrated e.g.
for using super speed. This patch adds a new phy_calibrate() method.
When the calibration should be used is dependent on actual chip.

In case of dwc3 on Exynos the calibration must happen after usb_add_hcd()
(while in host mode), because certain phy parameters like Tx LOS levels
and boost levels need to be calibrated further post initialization of xHCI
controller, to get SuperSpeed operations working. But an hcd must be
prepared first in order to pass it to usb_add_hcd(), so, in particular, dwc3
registers must be available first, and in order for the latter to happen
the phys must be initialized. This poses a chicken and egg problem if
the calibration were to be performed in phy_init(). To break the circular
dependency a separate method is added which can be called at a desired
moment after phy intialization.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>