spi: Kconfig: Create ISSI Kconfig entry

Add ISSI to Kconfig to make it selectable via menuconfig.
Also convert all current platforms.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/drivers/mtd/spi/Kconfig b/drivers/mtd/spi/Kconfig
index 4484cf8..98485b1 100644
--- a/drivers/mtd/spi/Kconfig
+++ b/drivers/mtd/spi/Kconfig
@@ -66,6 +66,11 @@
 	help
 	  Add support for various GigaDevice SPI flash chips (GD25xxx)
 
+config SPI_FLASH_ISSI
+	bool "ISSI SPI flash support"
+	help
+	  Add support for various ISSI SPI flash chips (ISxxx)
+
 config SPI_FLASH_MACRONIX
 	bool "Macronix SPI flash support"
 	help