Intrng: adjusting pic_init_secondary for further use cases

[ehem]

Issue is pic_init_secondary() was implemented for root PICs-only. The author's theory appears to have been the root calls the function on child PICs. At least one secondary PIC needs the function called, but the device-tree doesn't have it as a child of the root PIC. As such instead simply calling the function on all registered PICs appears to make more sense.