Allow MIPS INTRNG code to be built without FDT support.

This patch allows the newly imported INTRNG code to be built without necessarily
having FDT support in the kernel. This may be useful for some MIPS platforms
that wish to move to INTRNG, but not to FDT at the same time.

Basically all the code is already within ifdef's where FDT is concerned,
it's just the headers that aren't.

Submitted by: Stanislav Galabov <sgalabov@gmail.com>
Differential Revision: https://reviews.freebsd.org/D5249