RFC: migrate arm/arm/intrng.c -> kern/subr_intr.c in prep for MIPS using it

[Adrian Chadd]

Hi,

kan@ has been working on bringing in support for the CI20 board -
where he used the arm intrng code in his mips port.

So, I figured it is a good time to migrate the arm code into a generic
spot to be reused.

Here's my initial port:

http://people.freebsd.org/~adrian/arm/20151216-arm-intrng-generic-1.diff

It:

* moves the machdep bits into a separate file (arm/arm/machdep_intr.c);
* move intrng.c into sys/kern/subr_intr.c;
* rename the ARM_ -> INTR_, and arm_ -> intr_ symbols;
* fix up a variety of consumers of said code.

It's a WIP - I plan on doing the minimum needed to put it there so we
can use the code for other ports, and not try to solve the "how do we
integrate this cleanly into the existing interrupt code" problem.

This intr sharing code should make kan's job a lot easier. :)

Thanks!


-a