Multiplexing the pins

Hello

I have problem understanding how I should multiplex pins on the am5728 to get the corresponding pin output in the expansion headers, I know that in the reference manual are all the configuration for pins (CTRL_CORE_PAD_x), for example I want pin VIN3A_D20 to be uart6_RXD and VIN3A_D21 to be uart6_TXD, in the reference manual it corresponds to CTRL_CORE_PAD_GPMC_A4, I should change the bits 3:0 to 0x8 in order mux it, so in the x15 I have to decompile its dtb (in my case am57xx-beagle-x15-revc) to dts and modify it. This is where I’m stuck, I don’t know how to make that change. The idea I have is to make this node.

pinmux@1450
{

}
with the offset of the register 0x1450 but I’m not sure if this is okay or if its okay what should I declare inside

Regards
Michael