usb: dwc3: Add API to read back after register write

Write memory barrier can be used to ensure that the write buffer
on the processor has completed its operation before subsequent
operations can be started. However, it does not check the status
of the bus level write buffers. In case a device memory write
needs to complete an operation without waiting for next write to
same device memory region (which flushes previous write), it is
recommended to perform a read back right after write operation.
This eliminates the need to add memory barrier and also makes
sure bus level write buffer gets flushed.

Change-Id: I3af062be6eab65bb264f1a4d030672aa0c5f6b9a
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>