spi: kconfig: add soft spi Kconfig entry

Add the Kconfig entry for SOFT_SPI which uses gpio to simulate the
SPI signals. We use it for accessing 74x164 on some i.MX boards.
Signed-off-by: NPeng Fan <peng.fan@nxp.com>
Cc: Jagan Teki <jagan@openedev.com>
Cc: Stefano Babic <sbabic@denx.de>

drivers/spi/Kconfig

@@ -197,6 +197,12 @@ config OMAP3_SPI
 
 endif # if DM_SPI
 
+config SOFT_SPI
+	bool "Soft SPI driver"
+	help
+	 Enable Soft SPI driver. This driver is to use GPIO simulate
+	 the SPI protocol.
+
 config FSL_ESPI
 	bool "Freescale eSPI driver"
 	help