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: Jonas 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: Ralf Baechle <ralf@linux-mips.org>