device/device-lg-bullhead: disable 2 cpu's to prevent bootlooping

Quite a few Nexus 5X devices have the tendency to bootloop after a while of being on, iirc because it overheats. This has been a known issue for quite some time, and most Android ROM's for it have measures in place to prevent this, in the form of disabling some CPU cores (iirc "Big cores" or something like that). My Nexus 5X is also subject to this issue.

According to @craftyguy, resolving this is as easy as changing the boot command to not use those CPU cores. This PR does exactly that. It should probably be made optional for people with this device without that issue, but since I have no clue how to do that atm, this is the safest for now.