ti-gfx: systemd support

The startup script too complex to embed it completely into the
.service file, so it is left as a standalone script.

[Thomas:
 - use relative path for the symbolic link
 - slightly tweak the service Description string]

Signed-off-by: Alex Suykov <alex.suykov@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>