USB: fsl_udc_core: support device mode of MPC5121E DR USB Controller

Extend the FSL UDC driver to support MPC5121E DR USB Controller
operation in device mode. Add MPC5121E specific init/uninit
at probe and remove and isolate system interface register accesses
when running on MPC5121E SoC, as these registers are not available
on this platform. This patch relies on previous patch for supporting
big endian registers and descriptors access in the FSL UDC driver.
Additionally support endpoint FIFO status operation by providing
appropriate callback in endpoint ops structure.

Also flush cache for the req buffer used for GetStatus reply.
Without this, the correct reply to an endpoint GetStatus
is written to 'req', but doesn't make it out to the USB bus
since the buffer hasn't been flushed. This would cause the
USBCV Halt Endpoint test to fail (according to changelog in
Freescale LTIB driver code).

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Li Yang <leoli@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>