Remove the bcmmbox_write(BCMMBOX_CHANPM, ...) call in bcmmbox_attach().

Remove the bcmmbox_write(BCMMBOX_CHANPM, ...) call in bcmmbox_attach().

This command locks up my RaspberryPi 4 while it seems to have no effect
for other devices. First of all u-boot will already enable power for us
plus it seems that more is needed than just this write to be really
effective. Why my rpi4 locks up is still unclear but this is now a
pure u-boot issue.

Tested by kettenis@ on his working rpi4 and rpi2
OK kettenis@