x86: Fix rebooting on Dell Precision WorkStation T7400

Dell Precision WorkStation T7400 freezes on reboot unless
reboot=b is used.

Reference: https://qa.mandriva.com/show_bug.cgi?id=58017



Signed-off-by: Thomas Backlund <tmb@mandriva.org>
LKML-Reference: <4C1CC6E9.6000701@mandriva.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>