fbtft overlay is missing

[Impact]

To attach TFT touchscreens the pi-foundation kernel ships the fbtft devicetree overlay along with rpi-display.

While you can attach a stand-alone TFT display just fine when only using rpi-display to provide the drivers with hardcoded GPIO assignments, it gets extremely tricky to even enable the touchscreen input or to use such a TFT with other sensors attached since rpi-display does not allow any re-assignment of the GPIO pins in use.

The upstream fbtft driver can make use of the already included rpi-display drivers by defining "rpi-display" in its params. It also allows to freely re-assign the GPIO pins for backlight, D/C and Reset to give you enough flexibility to have the display co-exist with any attached sensors and touch input devices that do not have the ability to re-assign GPIO pins and that clash with the hardcoded numbering of rpi-display.
An example with freely assigned GPIOs can be seen in the upstream (rpi-foundation) overlay README file at:

https://github.com/raspberrypi/firmware/blob/master/boot/overlays/README#L884

Please include fbtft in the ubuntu kernels to allow a more flexible usage of SPI based TFT touchscreens on the Pi.

[Test case]

See above.

[Fix]

Backport commit 6a4525924c43 ("overlays: Add fbtft overlay") from rpi-5.15.y.

[Where Problems could occur]

Problems might show up when using the new DTB overlay on a Pi platform.