usb: phy: samsung: Common out the generic stuff

Moving register and structure definitions to header file,
and keeping the generic functions to be used across
multiple PHYs in common phy helper driver under SAMSUNG_USBPHY,
and moving USB 2.0 PHY driver under SAMSUNG_USB2PHY.

Also allowing samsung PHY drivers be built as modules.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>