usb: s3c-otg: Encapsulate PHY control

Encapsulate the Samsung PHY control and its register accesses
into the otg_phy_init() and otg_phy_off() functions.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Vince Bridgers <vbridger@altera.com>
Cc: Stefan Roese <sr@denx.de>
Acked-by: Pavel Machek <pavel@denx.de>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>