usb: s3c-otg: Allow custom gusbcfg

Allow passing in a custom configuration of the gusbcfg register
via platform data.

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>
Acked-by: Pavel Machek <pavel@denx.de>
Cc: Stefan Roese <sr@denx.de>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>