PM: domains: fix integer overflow issues in genpd_parse_state()

Currently, while calculating residency and latency values, right
operands may overflow if resulting values are big enough.

To prevent this, albeit unlikely case, play it safe and convert
right operands to left ones' type s64.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 30f60428 ("PM / Domains: Allow domain power states to be read from DT")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>