lg-bullhead: adapt for mainline kernel support

Hi everyone, this is my first merge request to pmaports. (sorry if things are not quite the way they're meant to be)

I'm continuing the work of @pevik on the support of MSM8992/MSM8994 devices by adding the missing code for mainline support of the Nexus 5x.

I haven't been able to test this on my device (due to the eMMC issue), hence the Draft status of this MR.