serial: imx: change name of configuration flag for AUTO mode

As "fsl,auto-mode" will only set Tx of UART to work in AUTO mode,
"fsl,uart-pio-datasize" only takes effect when "fsl,auto-mode" is set.
So change name of these two flags to "fsl,tx-auto-mode" and
"fsl,tx-auto-mode-threshold".
Also adds more detailed explaination of the usage for DMA mode related
DTS flags in
Documentation/devicetree/bindings/tty/serial/fsl-imx-uart.txt

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
serial: imx: change the logic of AUTO mode

Previously when UART is configured to work in AUTO mode, Rx will always
work in PIO mode, this makes AUTO mode can't fit for use case,
which requires TX in AUTO mode but RX in DMA mode.
Change the logic of AUTO mode, flag "fsl,tx-auto-mode" only configures
UART TX part to work in AUTO mode, won't affect RX part.
This means when both "fsl,enable-dma" and "fsl,tx-auto-mode"
are set in UART DT node, UART TX will work in AUTO mode and UART RX
will work in DMA mode.
If only "fsl,tx-auto-mode" is set in UART DT node, UART TX will work
in AUTO mode and UART RX will work in PIO mode.

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
serial: imx: set AUTO mode default threshold to half of FIFO size

If UART is configured to TX AUTO mode, but threshold value is not set,
instead of let TX always work in PIO mode, use half of FIFO size as
threshold value, so that TX can work in AUTO mode.

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
ARM: dts: imx6: remove tx-auto-mode-threshold configuration

Setting tx-auto-mode-threshold of 4096 doesn't make much sense
because the TX circular buffer size is one page (4096 bytes).
This means the tx-auto-mode uses PIO mode most of the time,
only when whole buffer is used for TX, DMA mode will be used.
Remove this configuration from all UART dts node, to let AUTO mode
work with default threshold value, which is half of UART FIFO size (16).

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>