Some customer cases need hardware partition and the GPIO need to be allocated by bank.
It causes resource tensive. And some time, one GPIO bank assigned to the M4 side 32 bits of one bank are more than enough. And some customer cases need to use 1 or 2 bit to as board identification for the A core side. Because of the hardware partition, A core could not directly access the M4 partition resource.
SCU has highest right to access any resource.
SC firmware for SCU has Remote Procedure Call (RPC) mechanism