PCI/ASPM: Always build aspm.c

PCI/ASPM: Always build aspm.c

Some ASPM-related tasks, such as save and restore of LTR and L1SS
capabilities, still need to be performed when CONFIG_PCIEASPM is not
enabled. To prepare for these changes, wrap the current code in aspm.c
with an #ifdef and always build the file.

Link: https://lore.kernel.org/r/20240128233212.1139663-2-david.e.box@linux.intel.com
[bhelgaas: split build change from function moves]
Link: https://lore.kernel.org/r/20240223205851.114931-3-helgaas@kernel.org
Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>