usb: s3c-hsudc: add basic runtime_pm calls

This will enable the system to check for activity of the usb gadget
and also in a later patch to control the usbphy power-domain.
When handling the power domain there, it will be possible to remove
another reference to architecture code.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>