mb/system76/rpl: bonw15: Update GPIOs

Go through the schematics and update GPIOs for the unit. In particular,
explicitly mark unconnected pins and pins without placed components as
not connected.

Change-Id: I5a81115850d7bf3ecabeae29058e86cea51ac390
Signed-off-by: Tim Crawford <[email protected]>

Author: crawfxrd