powerpc: Reword the "returning from prom_init" message

We get way too many bug reports that say "the kernel is hung in
prom_init", which stems from the fact that the last piece of output
people see is "returning from prom_init".

The kernel is almost never hung in prom_init(), it's just that it's
crashed somewhere after prom_init() but prior to the console coming up.

The existing message should give a clue to that, ie. "returning from"
indicates that prom_init() has finished, but it doesn't seem to work.
Let's try something different.

This prints:

  Quiescing Open Firmware ...
  Booting Linux via __start() ...

Which hopefully makes it clear that prom_init() is not the problem, and
although __start() probably isn't either, it's at least the right place
to begin looking.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Wistfully-Acked-by: Jeremy Kerr <jk@ozlabs.org>

arch/powerpc/kernel/prom_init.c

@@ -2898,7 +2898,7 @@ unsigned long __init prom_init(unsigned long r3, unsigned long r4,
 	 * Call OF "quiesce" method to shut down pending DMA's from
 	 * devices etc...
 	 */
-	prom_printf("Calling quiesce...\n");
+	prom_printf("Quiescing Open Firmware ...\n");
 	call_prom("quiesce", 0, 0);
 
 	/*
@@ -2910,7 +2910,7 @@ unsigned long __init prom_init(unsigned long r3, unsigned long r4,
 
 	/* Don't print anything after quiesce under OPAL, it crashes OFW */
 	if (of_platform != PLATFORM_OPAL) {
-		prom_printf("returning from prom_init\n");
+		prom_printf("Booting Linux via __start() ...\n");
 		prom_debug("->dt_header_start=0x%x\n", hdr);
 	}