sunxi: docs: Mention CONFIG_NAND requirement

Signed-off-by: Priit Laes <plaes@plaes.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>

board/sunxi/README.nand

@@ -20,9 +20,9 @@ pages.
 In order to accomodate that, we create a tool that will generate an
 SPL image that is ready to be programmed directly embedding the ECCs,
 randomized, and with the necessary bits needed to reduce the number of
-bitflips. The U-Boot build system, when configured for the NAND will
-also generate the image sunxi-spl-with-ecc.bin that will have been
-generated by that tool.
+bitflips. The U-Boot build system, when configured for the NAND (with
+CONFIG_NAND=y) will also generate the image sunxi-spl-with-ecc.bin
+that will have been generated by that tool.
 
 In order to flash your U-Boot image onto a board, assuming that the
 board is in FEL mode, you'll need the sunxi-tools that you can find at