x86: pcie: fallback to config via PIO

When probing for PCI-E device resources, it is possible that
configuration via MMIO is not available. This may caused by
BIOS or its settings. So when CONFIG_PCIE_MMIO_CFG=y, have
a fallback path to config devices via PIO. The inability to
config via MMIO has been observed on a couple UP Squared
boards.

Fixes #27339

Signed-off-by: Daniel Leung <daniel.leung@intel.com>