PM / OPP: Mark cpumask as const in dev_pm_opp_set_sharing_cpus()

dev_pm_opp_set_sharing_cpus() isn't supposed to update the cpumask
passed as its parameter, and so it should always have been marked
'const'.

Do it now.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>