CONFIG_PCI_MMCONFIG is not available for the selected architecture x86.
Result is shown for architecture x86_64

Support mmconfig PCI config space access

modulename: mmconfig.ko

configname: CONFIG_PCI_MMCONFIG

Linux Kernel Configuration
└─>Bus options (PCI etc.)
└─>Support mmconfig PCI config space access
In linux kernel since version 2.6.12 )
selects
CONFIG_ACPI_BOOT