MIPS: Kconfig: Avoid build errors by hiding USE_OF from the user.

b01da9f1 ("MIPS: Prune some target specific code out of prom.c") removed
the generic implementation of device_tree_init, breaking the kernel
build when manually selecting USE_OF.

Hide the config symbol so it can't be selected acidentially anymore.
Signed-off-by: NJonas Gorski <jonas.gorski@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: David Daney <david.daney@cavium.com>
Patchwork: https://patchwork.linux-mips.org/patch/4346/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -2371,12 +2371,10 @@ config SECCOMP
 	  If unsure, say Y. Only embedded should say N here.
 
 config USE_OF
-	bool "Flattened Device Tree support"
+	bool
 	select OF
 	select OF_EARLY_FLATTREE
 	select IRQ_DOMAIN
-	help
-	  Include support for flattened device tree machine descriptions.
 
 endmenu