PM / OPP: Make OPP invisible to users in Kconfig

The OPP code is an in kernel library selected by its users, there is no
no architecture code required to implement it and enabling it without a
user just increases the kernel size. Since the users select rather than
depend on it just remove the ability to directly set the option from
Kconfig.
Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Nishanth Menon <nm@ti.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/power/Kconfig

@@ -257,8 +257,7 @@ config ARCH_HAS_OPP
 	bool
 
 config PM_OPP
-	bool "Operating Performance Point (OPP) Layer library"
-	depends on ARCH_HAS_OPP
+	bool
 	---help---
 	  SOCs have a standard set of tuples consisting of frequency and
 	  voltage pairs that the device will support per voltage domain. This