The i.MX6 USB phy does not recognize disconnects of high speed
devices when the USBPHY_CTRL_ENHOSTDISCONDETECT is not set. The
phy does not work properly though when this bit is always set, so
implement the notify_(dis)concect() callbacks to set this bit
whenever a high speed device is connected and to clear it again
when the device is disconnected.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>