PM / OPP: Hide OPP configuration when SoCs do not provide an implementation

Since the OPP API is only useful with an appropraite SoC-specific
implementation there is no point in offering the ability to enable
the API on general systems. Provide an ARCH_HAS OPP Kconfig symbol
which masks out the option unless selected by an implementation.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Nishanth Menon <nm@ti.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

Documentation/power/opp.txt

@@ -37,6 +37,9 @@ Typical usage of the OPP library is as follows:
 SoC framework	-> modifies on required cases certain OPPs	-> OPP layer
 		-> queries to search/retrieve information	->
 
+Architectures that provide a SoC framework for OPP should select ARCH_HAS_OPP
+to make the OPP layer available.
+
 OPP layer expects each domain to be represented by a unique device pointer. SoC
 framework registers a set of initial OPPs per device with the OPP layer. This
 list is expected to be an optimally small number typically around 5 per device.

kernel/power/Kconfig

@@ -246,9 +246,13 @@ config PM_OPS
 	depends on PM_SLEEP || PM_RUNTIME
 	default y
 
+config ARCH_HAS_OPP
+	bool
+
 config PM_OPP
 	bool "Operating Performance Point (OPP) Layer library"
 	depends on PM
+	depends on ARCH_HAS_OPP
 	---help---
 	  SOCs have a standard set of tuples consisting of frequency and
 	  voltage pairs that the device will support per voltage domain. This