PM / Domains: Join state name and index in debugfs output

For low-power states, the state index is part of the state, hence join
them with a hyphen in the /sys/kernel/debug/pm_genpd/pm_genpd_summary
output. E.g. "off 0" becomes "off-0".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>