Add tinydm.service to tinydm.git

Let's add the systemd service here in this repository. The UID and GID should be a variable that the packager can set (as the default is different in pmOS than most other distros). I'll add it to the systemd-services package temporarily for now.

See also:

• https://github.com/dreemurrs-embedded/Pine64-Arch/blob/master/PKGBUILDS/sxmo/tinydm/tinydm.service