oneplus-dumpling: new device (OnePlus 5T)

Add support for the OnePlus 5T (oneplus-dumpling)

On top of just adding in support for the 5T I also:

• renamed linux-oneplus-cheeseburger -> linux-oneplus-msm8998 since it's shared between the regular 5 & 5T devices
• updated that downstream kernel to v4.4.217 (latest tag in upstream linux-stable 4.4.y as of 2020/03/27) because I can :)

I'm still not sure how I should adjust https://wiki.postmarketos.org/wiki/OnePlus_5_(oneplus-cheeseburger) properly as to have two devices under one page, or if that should even be done to begin with.

EDIT: Initial merge request for the OnePlus 5 (cheeseburger) is here if that can be useful in any way: pma!908