usb: s3c-otg: Rename struct s3c_usbotg_phy to dwc2_usbotg_phy

The driver is actually for the Designware DWC2 controller.
This patch renames struct s3c_usbotg_phy to struct dwc2_usbotg_phy
to make things more obvious and clear.

Signed-off-by: Marek Vasut <marex@denx.de>