sf: Add CONFIG_SF_DUAL_FLASH

This config will use for defining greater than single flash support.
currently - DUAL_STACKED and DUAL_PARALLEL.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
diff --git a/README b/README
index a0646c3..aea82be 100644
--- a/README
+++ b/README
@@ -2756,6 +2756,12 @@
 		Define this option to use the Bank addr/Extended addr
 		support on SPI flashes which has size > 16Mbytes.
 
+		CONFIG_SF_DUAL_FLASH		Dual flash memories
+
+		Define this option to use dual flash support where two flash
+		memories can be connected with a given cs line.
+		currently Xilinx Zynq qspi support these type of connections.
+
 - SystemACE Support:
 		CONFIG_SYSTEMACE