Document powerpc udbg-immortal

Back in 2006 the "udbg-immortal" kernel option has been introduced:

  > commit 3b5e905e
  > Author: Benjamin Herrenschmidt <benh@au1.ibm.com>
  > Date:   Wed Jun 7 12:06:20 2006 +1000
  >
  >  [PATCH] powerpc: Add udbg-immortal kernel option

...but I could not find it documented anywhere in the sources.
This patch adds it to Documentation/kernel-parameters.txt.

Signed-off-by: Christian Kujau <lists@nerdbynature.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>