I haven't gotten anything to work on the PinePhone.
However, I'd just like to note that the recently announced PinePhone Pro
uses a variant of the RK3399 SoC, which is the same SoC used in the
RockPro64 board, which seL4 has a target for.
This is the same SoC used in the PineBook Pro, which I was successfully
able to run sel4test on using the RockPro64 platform configuration.