MIPS: prepare for user enabling of CONFIG_OF

In preparation to allow users to enable DeviceTree without arch or
machine selecting it, we need to fix build errors on MIPS. When
CONFIG_OF is enabled, device_tree_init cannot be resolved. This is
trivially fixed by using CONFIG_USE_OF instead of CONFIG_OF for prom.h.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Acked-by: Grant Likely <grant.likely@lianro.org>
Cc: linux-mips@linux-mips.org