cpufreq: governor: Move abstract gov_attr_set code to seperate file

Move abstract code related to struct gov_attr_set to a separate (new)
file so it can be shared with (future) goverernors that won't share
more code with "ondemand" and "conservative".

No intentional functional changes.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
diff --git a/drivers/cpufreq/Kconfig b/drivers/cpufreq/Kconfig
index a7f4585..7fd8710 100644
--- a/drivers/cpufreq/Kconfig
+++ b/drivers/cpufreq/Kconfig
@@ -18,7 +18,11 @@
 
 if CPU_FREQ
 
+config CPU_FREQ_GOV_ATTR_SET
+	bool
+
 config CPU_FREQ_GOV_COMMON
+	select CPU_FREQ_GOV_ATTR_SET
 	select IRQ_WORK
 	bool