usb: ehci-mx6: introduce config for high active power pin

Add a new config CONFIG_MXC_USB_OTG_HACTIVE which configures the
OTG Power Pin to be high active. Low active is the reset value
of the affected configuration register, hence the config option
is named by the non-reset configuration.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>