[POWERPC] Document when printk is useable

When debugging early boot problems, it's common to sprinkle printk's
all over the place.  However, on 64-bit powerpc, this can lead to
memory corruption if done too early due to the PACA pointer and
lockdep core not being initialized.

This adds some comments to early_setup() that document when it is
safe to do so in order to save time for whoever has to debug that
stuff next.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>