fairphone-fp1: add mainline deviceinfo variables
While it doesn't make sense to package a mainline kernel for FP1 (only
UART and basic SoC components supported), it's good to include the dtb
name in deviceinfo so external tools can use it.
Background: The script I use for mainline kernel development (https://github.com/z3ntu/linux-mainline-scripts/blob/master/boot-from-sourcetree.sh) relies on pmaports deviceinfo to give details about the device like dtb name.