cpumask: Rephrase comments for cpumask_any*() APIs

cpumask: Rephrase comments for cpumask_any*() APIs

The cpumask_any*() APIs comment states that it returns a "random" cpu
within the given cpumask. However it's not actually random as random
itself stands a meaning for uniform distribution.

cpumask_any*() APIs are a naming convention for the caller to states that
it doesn't care which CPU it gets, so change "random" to "arbitrary" would
be more appropriate.

CC: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>