x86 setup: add note to use edd=off on EDD probing hangs

Tell the user to specify edd=off in the case of EDD probing hangs.
Per LKML discussion.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>