Add BeagleBone AI P8 header pinmux nodes

Created by: lorforlinux

This change is a step towards making the capes compatible with both BBBlack and BBAi. I have included all the P8 header pin muxing nodes. Some of the pins that are attached to eQEP and PWM peripherals of AM572x require editing. I have included /* ?? ... */ at the end of nodes that are not final and require editing (which I will do soon). some functionalities are present on a different pin of AM572x than AM335x and we have to shift them accordingly.

Example: eqep1_strobe available on 
P9_21 of AM572X 
P8_32 of AM335X

I will create a new commit for P9 header before editing the nodes of P8 header.

@jadonk @RobertCNelson please review the commit and let me know the required changes if any.