[ Upstream commit 49ad712a ]

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.cg...