usb: phy: generic: Don't use regulator framework for RESET line

Modelling the RESET line as a regulator supply wasn't a good idea
as it kind of abuses the regulator framework and also makes adaptation
code more complex.

Instead, manage the RESET gpio line directly in the driver. Update
the device tree binding information.

This also makes us easy to migrate to a dedicated GPIO RESET controller
whenever it becomes available.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>