USB: add USB EHCI support for MPC5121 SoC

Extends FSL EHCI platform driver glue layer to support
MPC5121 USB controllers. MPC5121 Rev 2.0 silicon EHCI
registers are in big endian format. The appropriate flags
are set using the information in the platform data structure.
MPC83xx system interface registers are not available on
MPC512x, so the access to these registers is isolated in
MPC512x case. Furthermore the USB controller clocks
must be enabled before 512x register accesses which is
done by providing platform specific init callback.

The MPC512x internal USB PHY doesn't provide supply voltage.
For boards using different power switches allow specifying
DRVVBUS and PWR_FAULT signal polarity of the MPC5121 internal
PHY using "fsl,invert-drvvbus" and "fsl,invert-pwr-fault"
properties in the device tree USB nodes. Adds documentation
for this new device tree bindings.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>