spi: Add Designware SPI controller Kconfig entry

Add DWC SPI controller Kconfig entry.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@opensource.altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Acked-by: Pavel Machek <pavel@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Stefan Roese <sr@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Vince Bridgers <vbridger@opensource.altera.com>

drivers/spi/Kconfig

@@ -35,3 +35,11 @@ config SANDBOX_SPI
 			sandbox,filename = "spi.bin";
 		};
 	};
+
+config DESIGNWARE_SPI
+	bool "Designware SPI driver"
+	depends on DM_SPI
+	help
+	  Enable the Designware SPI driver. This driver can be used to
+	  access the SPI NOR flash on platforms embedding this Designware
+	  IP core.