Some user space applications are designed around the ability to perform
atomic operations on 64 bit values.  Since this is natively possible
only with ARMv6k and above, let's provide a new kuser helper to perform
the operation with kernel supervision on pre ARMv6k hardware.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Tested-by: Dave Martin <dave.martin@linaro.org>

Digging into some assembly file in order to get information about the
kuser helpers is not that convivial.  Let's move that information to
a better formatted file in Documentation/arm/ and improve on it a bit.

Thanks to Dave Martin <dave.martin@linaro.org> for the initial cleanup and
clarifications.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Acked-by: Dave Martin <dave.martin@linaro.org>