iwlagn: add a iwl_pci.[ch] files that will contain all PCI specific code

Move some PCI functionality to the new iwl_pci.[ch] files:
* the PCI_DEVICE_TABLE
* the pci_driver struct definition
* the PCI probe / remove functions
* the PCI suspend / resume functions

All these functions are now split: the trigger comes from the PCI layer which
calls to the bus generic code located in the other files.

This is the beginning only. There are still a lot of PCI related code needs
to be gathered.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>