MIPS: Fix build with CONFIG_OF=y for non OF-enabled targets

Commit 01306aea ("MIPS: prepare for user enabling of CONFIG_OF")
changed the guards in asm/prom.h from CONFIG_OF to CONFIG_USE_OF, but
missed the actual function declarations in kernel/prom.c, which have
additional dependencies.

Fixes the following build error:

  CC      arch/mips/kernel/prom.o
arch/mips/kernel/prom.c: In function '__dt_setup_arch':
arch/mips/kernel/prom.c:54:2: error: implicit declaration of function 'early_init_dt_scan' [-Werror=implicit-function-declaration]
  if (!early_init_dt_scan(bph))
  ^

Fixes: 01306aea ("MIPS: prepare for user enabling of CONFIG_OF")
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Acked-by: Rob Herring <robh@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: Grant Likely <grant.likely@linaro.org>
Patchwork: https://patchwork.linux-mips.org/patch/10741/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>