Well, It looks like a TSOT23-5 device. And your choices are pretty limited (which is a good news). If I have to guess, it could be a voltage regulator or an Opamp given it's proximity to the ADC (AK5357). If you trace out the passives around U12 you probably can at least tell what type of a device it is (e.g. a regulator or an op-amp)...