USB: ehci_fsl update for MPC831x support

For MPC831x support, change the ehci-fsl driver to preserve
bits set in platform code.  Add a common CONFIG_USB_EHCI_FSL
to indicate presence of Freescale EHCI SOC.  Add FSL_USB2_DR_OTG
operating mode support, thus both host and device can work for the
mini-ab receptacle.  Note: this doesn't enable OTG protocol
support.

Signed-off-by: Li Yang <leoli@freescale.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>