[TF-A] RPi4 and CPU hotplugging

Hi Andre,

I'm trying to get CPU offlining work for Linux on the RPi4. In fact, this is working already with current ATF master and the Raspberry kernel 4.19.85. Not it would be "nice" if onlining worked as well:

[ 94.959294] CPU1: shutdown [ 94.959311] psci: CPU1 killed. [ 106.750960] CPU1: failed to come online [ 106.755425] CPU1: failed in unknown state : 0x0

Is this a known limitation? Or am I using the wrong kernel (obviously, it's a downstream one ;) )?

Thanks, Jan

PS: Current RPi firmware requires an explicit "armstub=armstub8.bin" in config.txt, no automatic pickup.