MLK-10085-4 usb: doc: chipidea: imx: add imx6-usb-charger-detection property

It is used to indicate whether we use SoC's usb charger
detection or not. Besides, we add anatop phandle since
we need to use anatop register to do most of charger detect operations.

Signed-off-by: Peter Chen <peter.chen@freescale.com>