dt-bindings: mfd: atmel-usart: add DMA bindings for USART in SPI mode

The bindings for DMA are now common for both drivers of the USART
IP.

The node given as an example for USART in SPI mode has been updated in
order to include DMA bindings.

Signed-off-by: Radu Pirea <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/mfd/atmel-usart.txt

···
 - cs-gpios: chipselects (internal cs not supported)
 - atmel,usart-mode : Must be <AT91_USART_MODE_SPI> (found in dt-bindings/mfd/at91-usart.h)
 
+Optional properties in serial and SPI mode:
+- dma bindings for dma transfer:
+	- dmas: DMA specifier, consisting of a phandle to DMA controller node,
+		memory peripheral interface and USART DMA channel ID, FIFO configuration.
+		The order of DMA channels is fixed. The first DMA channel must be TX
+		associated channel and the second one must be RX associated channel.
+		Refer to dma.txt and atmel-dma.txt for details.
+	- dma-names: "tx" for TX channel.
+		     "rx" for RX channel.
+		     The order of dma-names is also fixed. The first name must be "tx"
+		     and the second one must be "rx" as in the examples below.
+
 Optional properties in serial mode:
 - atmel,use-dma-rx: use of PDC or DMA for receiving data
 - atmel,use-dma-tx: use of PDC or DMA for transmitting data
 - {rts,cts,dtr,dsr,rng,dcd}-gpios: specify a GPIO for RTS/CTS/DTR/DSR/RI/DCD line respectively.
   It will use specified PIO instead of the peripheral function pin for the USART feature.
   If unsure, don't specify this property.
-- add dma bindings for dma transfer:
-	- dmas: DMA specifier, consisting of a phandle to DMA controller node,
-		memory peripheral interface and USART DMA channel ID, FIFO configuration.
-		Refer to dma.txt and atmel-dma.txt for details.
-	- dma-names: "rx" for RX channel, "tx" for TX channel.
 - atmel,fifo-size: maximum number of data the RX and TX FIFOs can store for FIFO
   capable USARTs.
 - rs485-rts-delay, rs485-rx-during-tx, linux,rs485-enabled-at-boot-time: see rs485.txt
···
 		interrupts = <12 IRQ_TYPE_LEVEL_HIGH 5>;
 		clocks = <&usart0_clk>;
 		clock-names = "usart";
+		dmas = <&dma0 2 AT91_DMA_CFG_PER_ID(3)>,
+		       <&dma0 2 (AT91_DMA_CFG_PER_ID(4) | AT91_DMA_CFG_FIFOCFG_ASAP)>;
+		dma-names = "tx", "rx";
 		cs-gpios = <&pioB 3 0>;
 	};