powerpc: Update a BKL related comment

The commit 5e3d20a6 remove bkl from startup code so setup_arch() it isn't called
with bkl held anymore. Update the comment on top of that function.
Fix also a typo.

This work was supported by a hardware donation from the CE Linux Forum.

Signed-off-by: Alessio Igor Bogani <abogani@texware.it>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>