serial: 8250: dw: Add support for DMA flow controlling devices

DW based controllers like the one on Renesas RZ/N1 must be programmed as
flow controllers when using DMA.

* Table 11.45 of the system manual, "Flow Control Combinations", states
  that using UART with DMA requires setting the DMA in the peripheral
  flow controller mode regardless of the direction.

* Chapter 11.6.1.3 of the system manual, "Basic Interface Definitions",
  explains that the burst size in the above case must be configured in
  the peripheral's register DEST/SRC_BURST_SIZE.

Experiments shown that upon Rx timeout, the DMA transaction needed to be
manually cleared as well.

Co-developed-by: Miquel Raynal <[email protected]>
Reviewed-by: Andy Shevchenko <[email protected]>
Signed-off-by: Phil Edworthy <[email protected]>
Signed-off-by: Miquel Raynal <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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