x86: Add quirk to make Apple MacBook5,2 use reboot=pci

The latest Apple MacBook (MacBook5,2) doesn't reboot successfully
under Linux; neither the EFI reboot method nor the default method
using the keyboard controller works (the system just hangs and doesn't
reset).  However, the method using the "PCI reset register" at 0xcf9
does work.

This adds a quirk to detect this machine via DMI and force the
reboot_type to BOOT_CF9.  With this it reboots successfully without
requiring a command-line option.  Note that the EFI code forces
reboot_type to BOOT_EFI when the machine is booted via EFI, but this
overrides that since the core_initcall runs after the EFI
initialization code.

Signed-off-by: Paul Mackerras <paulus@samba.org>
LKML-Reference: <19062.56420.501516.316181@cargo.ozlabs.ibm.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>