oneplus-bacon: Enable mainline kernel

The wiki page of oneplus-bacon says it's compatible with mainline kernel, but the device-oneplus-bacon only has the downstream kernel.
Since the dts for oneplus-bacon qcom-msm8974-oneplus-bacon it's already present in linux-postmarketos, I've updated the APKBUILD and device info to enable the use of mainline kernel.

I've tested the phone with mainline and it boots and usb-ethernet works, the screen stays on the oneplus logo.

I know oneplus-bacon is currently unmaintained, I would be happy to become a maintainer!