something is missing on schematic of BB or LCD7

The AIN[7:0] have dual functionality. they can perform as stand along Analog to Digital Converters or function as 4-wire,5-wire or 8-wire resistive touch screen controllers. you can download the technical reference manual (also known as a TRM) for the AM335x from this url - (current revision listed as ****AM335x ARM Cortex-A8 Microprocessors (MPUs) Technical Reference Manual (Rev. H) )

the section describing the resistive touch screen controller begins on page 1022 as Chapter 12. there is a nice block diagram on page 1028 figure 12-2, which shows how the muxes allow the positive and negative references to be driven on each of the resistive pairs during sampling.