dt-bindings: Add binding for atmel-usart in SPI mode

* Atmel Universal Synchronous Asynchronous Receiver/Transmitter (USART)

Required properties:

Required properties for USART:

- compatible: Should be "atmel,<chip>-usart" or "atmel,<chip>-dbgu"

  The compatible <chip> indicated will be the first SoC to support an

  additional mode or an USART new feature.

	Required elements: "usart"

- clocks: phandles to input clocks.

Optional properties:

Required properties for USART in SPI mode:

- #size-cells      : Must be <0>

- #address-cells   : Must be <1>

- 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 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.

		dma-names = "tx", "rx";

		atmel,fifo-size = <32>;

	};

- SPI mode:

	#include <dt-bindings/mfd/at91-usart.h>

	spi0: spi@f001c000 {

		#address-cells = <1>;

		#size-cells = <0>;

		compatible = "atmel,at91rm9200-usart", "atmel,at91sam9260-usart";

		atmel,usart-mode = <AT91_USART_MODE_SPI>;

		reg = <0xf001c000 0x100>;

		interrupts = <12 IRQ_TYPE_LEVEL_HIGH>;

		clocks = <&usart0_clk>;

		clock-names = "usart";

		cs-gpios = <&pioB 3 0>;

	};

/* SPDX-License-Identifier: GPL-2.0 */

/*

 * This header provides macros for AT91 USART DT bindings.

 *

 * Copyright (C) 2018 Microchip Technology

 *

 * Author: Radu Pirea <radu.pirea@microchip.com>

 *

 */

#ifndef __DT_BINDINGS_AT91_USART_H__

#define __DT_BINDINGS_AT91_USART_H__

#define AT91_USART_MODE_SERIAL	0

#define AT91_USART_MODE_SPI	1

#endif /* __DT_BINDINGS_AT91_USART_H__ */