USB: fix build of FSL MPH DR OF platform driver

When only FSL USB gadget driver is selected in the kernel
configuration the MPH DR OF driver for creation of FSL
USB platform devices from device tree won't be built.
As a result no USB platform devices for MPH DR USB controller
will be created at run time and no probing will be done in
the fsl_udc_core driver.

Add an entry to the Makefile to build the MPH DR OF
platform driver if CONFIG_USB_FSL_MPH_DR_OF is defined.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>