serial: 8250_mid: add support for DMA engine handling from UART MMIO

The platforms that have this UART, but that don't have
separate PCI device for the DMA Engine, need to create the
HSU DMA Engine device separately.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>