Bug#991333: Backport Raspberry PI POE hat fan support from mainline

[Mathieu Chouquet-Stringer <me+debian@mathieu.digital>]

Package: linux-image-arm64
Version: 5.10.40-1~bpo10+1

Hello,

A raspberry pi can be powered with a poe hat
(https://www.raspberrypi.org/products/poe-hat/). Until recently the
fan driver for the hat wasn't in mainline but that changes when support
got merged into 5.13:
https://lore.kernel.org/linux-clk/20210312122454.24480-1-nsaenzjulienne@suse.de/

Here's the relevant commit:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=37f00ab4a003f371f81e0eae76cf372f06dec780

Without this driver, the fan doesn't spin at all and the raspberry runs
from hot to very hot (to the point I can make it overheat by loading it
too much).

Do you think it'd be doable to backport it to 5.10?

Regards,

-- 
Mathieu Chouquet-Stringer
            The sun itself sees not till heaven clears.
	             -- William Shakespeare --