meson-s4t7: add systemd service to enable fan in automatic mode

Description

The service is only enabled by default on Vim4 as that board heats
a lot and a fan is absolutely necessary. Service will be available
on vim1s as well and if someone has purchased the fan for it, they
would be able to enable it themselves.

The service is only added for legacy kernel right now as I am not
sure if the upcoming 5.15 kernel will have same sysfs interface for
fan.

https://github.com/armbian/build/commit/c0e2bb910c4e2ad5d0db36707569fcc6734afe55