spi: at91-usart: Add driver for at91-usart as SPI

This is the driver for at91-usart in spi mode. The USART IP can be configured
to work in many modes and one of them is SPI.

The driver was tested on sama5d3-xplained and sama5d4-xplained boards with
enc28j60 ethernet controller as slave.
Signed-off-by: Radu Pirea <radu.pirea@microchip.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviwed-by: Mark Brown <broonie@kernel.org>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/spi/Kconfig

@@ -83,6 +83,14 @@ config SPI_ATMEL
 	  This selects a driver for the Atmel SPI Controller, present on
 	  many AT91 ARM chips.
 
+config SPI_AT91_USART
+	tristate "Atmel USART Controller SPI driver"
+	depends on (ARCH_AT91 || COMPILE_TEST)
+	depends on MFD_AT91_USART
+	help
+	  This selects a driver for the AT91 USART Controller as SPI Master,
+	  present on AT91 and SAMA5 SoC series.
+
 config SPI_AU1550
 	tristate "Au1550/Au1200/Au1300 SPI Controller"
 	depends on MIPS_ALCHEMY

drivers/spi/Makefile

@@ -16,6 +16,7 @@ obj-$(CONFIG_SPI_LOOPBACK_TEST)		+= spi-loopback-test.o
 obj-$(CONFIG_SPI_ALTERA)		+= spi-altera.o
 obj-$(CONFIG_SPI_ARMADA_3700)		+= spi-armada-3700.o
 obj-$(CONFIG_SPI_ATMEL)			+= spi-atmel.o
+obj-$(CONFIG_SPI_AT91_USART)		+= spi-at91-usart.o
 obj-$(CONFIG_SPI_ATH79)			+= spi-ath79.o
 obj-$(CONFIG_SPI_AU1550)		+= spi-au1550.o
 obj-$(CONFIG_SPI_AXI_SPI_ENGINE)	+= spi-axi-spi-engine.o