usb: gadget: s3c-hsotg: enable build for other platforms

Remove unused Samsung-specific machine include and Kconfig
dependency on S3C.

Signed-off-by: Matt Porter <mporter@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>