x86/doc: mini-howto for using earlyprintk=dbgp

[ mingo: small edits and extensions. ]

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Greg KH <gregkh@suse.de>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Sarah Sharp <sarah.a.sharp@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <49AF18B7.4050305@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>