There are no ARM with complete open source because they all have the GPU integrated. The Mali is probably the closest to open due to the LIMA project with the Freedreno project backing Qualcomm's Adreno (and maybe work with the Freescale mx5x) getting access to the sources isn't particularly hard (but does require an NDA) but you still won't get the most important part, which is the source to the shader compiler library in there