usb:hsotg:samsung: Extract core initialization function

The s3c_hsotg_core_init function has been added to exclude
code responsible for Samsung's SoCs USB core initialization.
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>