I have successfully bound my controller to lakka, every key is mapped to its equivalent in RetroPad… but what keys is for what in playstation cores? A for O (Circle) or X (Cross)? Y for Square??? I know that I can figure it out myself but I would like to have something to refer to… I have checked the libretro wiki but nothing useful there 
The retropad is laid out like a ps3/360 pad, and the psx/psp bindings are in physically the same place as the corresponding retropad buttons. You can use the pic here as a guide: https://wiki.libretro.com/index.php?title=RetroPad