serial: 8250_dma: add timer for dma receive

serial: 8250_dma: add timer for dma receive

For rockchip serial, received data available and character timeout
interrupts are both enabled by IER[0]. Then when there is data in
the FIFO, received data available interrupt will occurd frequently.
So we must disable it, but which may disable the character timeout
interrput. Then it is useful to add a timer to report the data received
in dma buffer every 10 microsecond.

Change-Id: I6530b17800435b288a7309bb5998176decb94297
Signed-off-by: Huibin Hong <huibin.hong@rock-chips.com>