x86/reboot: Blacklist Dell OptiPlex 990 known to require PCI reboot

Dell OptiPlex 990 is known to require PCI reboot, so add it to
the reboot blacklist in pci_reboot_dmi_table[].

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Link: http://lkml.kernel.org/r/201111160019.51303.rjw@sisk.pl
Signed-off-by: Ingo Molnar <mingo@elte.hu>