serial: mxs: enable the DMA only when the RTS/CTS is valid

The original DMA support works only when RTS/CTS is enabled.
(see the "e8001632 serial: mxs-auart: add the DMA support for mx28")
But after several patches, DMA support has lost this limit.
(see the "bcc20f9e serial: mxs-auart: move to use generic DMA helper")

So an UART without the RTS/CTS lines may also enables the DMA support,
in which case the UART may gets unpredictable results.

This patch adds an optional property for the UART DT node
which indicates the UART has RTS and CTS lines, and it also means you
enable the DMA support for this UART.

This patch also adds a macro MXS_AUART_RTSCTS, and uses it to check
RTS/CTS before we enable the DMA for the UART.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Huang Shijie <shijie8@gmail.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>