device-pine64-pinebookpro: Enable s2idle in elogind

Right now, it appears elogind is trying to enter 'deep' sleep on the Pinebook Pro by default, which does not seem to be possible currently. This results in the device being frozen whenever you close the lid as in #2182.

Initially it seemed like a good idea to simply disable suspend altogether, but it appears s2idle works fine. This in theory will offer at least some power savings as it disables some peripherals such as WiFi. It also turns off the display when the lid is closed, which isn't possible for elogind to do with suspend disabled.