tlp.service: Set type to simple

no one service depend on tlp service while boot
according to http://freedesktop.org/wiki/Software/systemd/Optimizations/
Quote:
23. The usual housekeeping: get rid of shell-based services (i.e. SysV init scripts), replace them with unit files. Don't make use of Type=forking and ordering dependencies if possible, use socket activation with Type=simple instead. This allows drastically better parallelized start-up for your services. Also, if you cannot use socket activation, at least consider patching your services to support Type=notify in place of Type=forking. Consider making seldom used services activated on-demand (for example, printer services), and start frequently used services already at boot instead of delaying them until they are used.

simple tipe, allow systemd not stop boot, while service a boot and do start of service in backgroud.