edac: sb_edac: Let the driver depend on PCI_MMCONFIG

This driver needs to access PCIe Extended Configuration Space
Registers (0x100~0xfff), to correctly access those registers, we need
to enable PCI_MMCONFIG option. Since this option is not enabled for
X86_64 by default, we let the driver depend on it to prevent users
forgetting to enable this option.

Signed-off-by: Hui Wang <jason77.wang@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>