dts: Kconfig: Fix help for SPL_OF_CONTROL

As initially this feature was implemented as a negative CONFIG and
later it was redesigned to be positive the help text should be
updated to reflect this change.

This commit updates the help text to match the current implementation.
Signed-off-by: NWalter Lozano <wlozano@collabora.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>

dts/Kconfig

@@ -44,7 +44,7 @@ config SPL_OF_CONTROL
 	depends on SPL && OF_CONTROL
 	help
 	  Some boards use device tree in U-Boot but only have 4KB of SRAM
-	  which is not enough to support device tree. Enable this option to
+	  which is not enough to support device tree. Disable this option to
 	  allow such boards to be supported by U-Boot SPL.
 
 config TPL_OF_CONTROL