platform/x86: Add Intel AtomISP2 dummy / power-management driver

The Image Signal Processor found on Cherry Trail devices is brought up in
D0 state on devices which have camera sensors attached to it. The ISP will
not enter D3 state again without some massaging of its registers beforehand
and the ISP not being in D3 state blocks the SoC from entering S0ix modes.

There was a driver for the ISP in drivers/staging but that got removed
again because it never worked. It does not seem likely that a real
driver for the ISP will be added to the mainline kernel anytime soon.

This commit adds a dummy driver which contains the necessary magic from
the staging driver to powerdown the ISP, so that Cherry Trail devices where
the ISP is used will properly use S0ix modes when suspended.

Together with other recent S0ix related fixes this allows S0ix modes to
be entered on e.g. a Chuwi Hi8 Pro and a HP x2 210.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=196915
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/platform/x86/Kconfig

@@ -1274,6 +1274,18 @@ config I2C_MULTI_INSTANTIATE
 	  To compile this driver as a module, choose M here: the module
 	  will be called i2c-multi-instantiate.
 
+config INTEL_ATOMISP2_PM
+	tristate "Intel AtomISP2 dummy / power-management driver"
+	depends on PCI && IOSF_MBI && PM
+	help
+	  Power-management driver for Intel's Image Signal Processor found on
+	  Bay and Cherry Trail devices. This dummy driver's sole purpose is to
+	  turn the ISP off (put it in D3) to save power and to allow entering
+	  of S0ix modes.
+
+	  To compile this driver as a module, choose M here: the module
+	  will be called intel_atomisp2_pm.
+
 endif # X86_PLATFORM_DEVICES
 
 config PMC_ATOM