Blackfin: split out async setup

We really only need to tweak the async banks in the initcode if the
processor is booting out of it, otherwise we can wait until later
on in the CPU booting setup.

This also makes testing in the sim and early bring up over JTAG work
much smoother when the initcode gets bypassed.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>