x86: Align x86_64 PCI_MMCONFIG with 32-bit variant

Allow to enable PCI_MMCONFIG when only SFI is present and make this option
default on. This will help consolidating both into one Kconfig statement.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: jailhouse-dev@googlegroups.com
Cc: linux-pci@vger.kernel.org
Cc: virtualization@lists.linux-foundation.org
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lkml.kernel.org/r/a2faf78c54f340f5549149e8b679c95950dae83d.1520408357.git.jan.kiszka@siemens.com