Derek Molloy has published two very nice tables on github, with everything you need. Just do a web search for BeagleboneBlackP9HeaderTable or …P8… and you will find them.
From that table, P9.17 = PIN # 87, addr = 0x44E1095C, offset = 0x15C, in mode7 = gpio[5]
cat /sys/kernel/debug/pinctrl/44e10800.pinmux/pins to see all your current settings
Don’t forget the 4 ma current limit for GPIOs! Too low for even an LED.
I wrote to you in another thread that even without compiling the DT Overlay file that should have set the P9.15 to mode 7, I managed to turn on/off and output of P9.15 as if it was GPIO.