[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>

arch/powerpc/kernel/setup_64.c

@@ -170,6 +170,8 @@ void __init setup_paca(int cpu)
 
 void __init early_setup(unsigned long dt_ptr)
 {
+	/* -------- printk is _NOT_ safe to use here ! ------- */
+
 	/* Fill in any unititialised pacas */
 	initialise_pacas();
 
@@ -185,6 +187,8 @@ void __init early_setup(unsigned long dt_ptr)
 	/* Initialize lockdep early or else spinlocks will blow */
 	lockdep_init();
 
+	/* -------- printk is now safe to use ------- */
+
  	DBG(" -> early_setup(), dt_ptr: 0x%lx\n", dt_ptr);
 
 	/*