ENGR00287992-1 power: imx6: add imx6 USB charger detection

ENGR00287992-1 power: imx6: add imx6 USB charger detection

Add imx6 USB charger detection, the vbus supplier will create and
remove struct usb_charger, and notify vbus connect and disconnect
event. The detail USB charger detection flow is at: "i.MX6 RM,
Chapter Universal Serial Bus 2.0 Integrated PHY (USB-PHY),
Charger detection, Charger detection software flow".

Since imx6 only has charger detection function, and no charging
current function is existed. It the user wants the detection abilities
from SoC, it can use this detection method
(add imx6-usb-charger-detection at dts). If the charger IC
already has USB charger detection function, and the user wants
to use the detection method from charger IC, please do not add
imx6-usb-charger-detection property at dts.

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