phy: Add set_mode callback

The initial use for this is for PHYs that have a mode related to USB OTG.
There are several SoCs (e.g. TI OMAP and DA8xx) that have a mode setting
in the USB PHY to override OTG VBUS and ID signals.

Of course, the enum can be expaned in the future to include modes for
other types of PHYs as well.

Suggested-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: David Lechner <david@lechnology.com>