Is there any example available of a working device-tree using a camera (any OVxxxx) for the 4.9 kernel in XIP with single QSPI ?
TBH I tried to backport it to the 4.9 kernel but I gave up a few hours after that, lot of things changed and it make the backport too heavy (for me). But I guess it will be much more easier to backport it to the 4.14 kernel. Regarding the 4.15 kernel I only had to add and patch AXFS to the kernel. I don't think I had to modify something else (except devicetree) Maybe I reused some ASM files (startup code) from the 4.9 to solve some compilations errors ? I didn't wrote down all the changes I've made.
To be honest, I think it will be better for you to switch to a 4.17 kernel. The reworked driver has been released, see git.kernel.org/.../renesas-ceu.c As I said previously in this post, backporting this driver to the 4.9 kernel will be complicated. Maybe it will be easier to backport it to the 4.14 kernel maintain by Renesas but It may take some time.