ARM: zynq defconfig: Be systemd ready

The Open Source Tea Party[1] is gaining more and more support and
sytemd[2] seems to become the default init system in a lot of
distros[3-5]. Hence, add the minimum kernel side requirements[6]
for systemd to the Zynq defconfig.

[1]: http://www.markshuttleworth.com/archives/1295
[2]: http://www.freedesktop.org/wiki/Software/systemd/
[3]: http://en.wikipedia.org/wiki/Systemd#Adoption
[4]: http://lwn.net/Articles/585363/
[5]: http://www.markshuttleworth.com/archives/1316
[6]: http://cgit.freedesktop.org/systemd/systemd/tree/README#n39



Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>