of: flip CONFIG_SPL_DISABLE_OF_CONTROL into CONFIG_SPL_OF_CONTROL

As we discussed a couple of times, negative CONFIG options make our
life difficult; CONFIG_SYS_NO_FLASH, CONFIG_SYS_DCACHE_OFF, ...
and here is another one.

Now, there are three boards enabling OF_CONTROL on SPL:
 - socfpga_arria5_defconfig
 - socfpga_cyclone5_defconfig
 - socfpga_socrates_defconfig

This commit adds CONFIG_SPL_OF_CONTROL for them and deletes
CONFIG_SPL_DISABLE_OF_CONTROL from the other boards to invert
the logic.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/dts/Kconfig b/dts/Kconfig
index 09cfefb..d72a909 100644
--- a/dts/Kconfig
+++ b/dts/Kconfig
@@ -14,9 +14,9 @@
 	  This feature provides for run-time configuration of U-Boot
 	  via a flattened device tree.
 
-config SPL_DISABLE_OF_CONTROL
-	bool "Disable run-time configuration via Device Tree in SPL"
-	depends on OF_CONTROL
+config SPL_OF_CONTROL
+	bool "Enable run-time configuration via Device Tree in SPL"
+	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