Fast cycle counter for ARM chips with SCC - patch for review.

[Bakul Shah]

FYI: BCM2835 has a built in h/w RNG (using a reverse biased transistor as an entropy source). Access to it was enabled in the RPi firmware around Jan 30, 2013. FWIW, it passes rngtest as well as FreeBSD's /dev/random (on x86/amd64) does.
For the spec. see http://pastehtml.com/view/crkxyohmp.rtxt
Referenced from this thread: http://www.raspberrypi.org/forums/viewtopic.php?f=29&t=19334&p=273944#p273944

> On May 10, 2014, at 10:39 AM, Mark R V Murray <markm at FreeBSD.org> wrote:
> 
> Hi *
> 
> This patch makes the ARM6 kernels that have an SCC coprocessor (RPI and WANDBOARD have them) get a MUCH better implementation of get_cyclecount(9), but not a perfect one. The incrementing rate is good (+- 1 per instruction), but its only 32 bits. Later, if there is interest, I may wish to fix that with an overflow interrupt, but for now its easily good enough for the kernel entropy harvesting service. Also, its MUUUCH more efficient; a simple read rather can calling the internal kernel binuptime(9) clock.
> 
> Comments, please? I’m keen to commit.
> 
> M
> -- 
> Mark R V Murray
> <arm_ccnt_counter.diff>
>