usb: gadget: atmel_usba_udc: condition clocks to vbus state

If USB PLL is not necessary for other USB drivers (e.g. OHCI and EHCI)
we will reduce power consumption by switching off the USB PLL if no USB
Host is currently connected to this USB Device.

We are using Vbus GPIO signal to detect Host presence. If Vbus signal is
not available then the device stays continuously clocked.

Signed-off-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>