x86: Add PCI support

This patch adds the pci.c and pci.h files, which support access to PCI
configuration registers through a function interface.  It defines the
PCI configuration register access I/O port addresses and the
pci_config_addr union and structure to assist in specifying addresses
of PCI configuration registers.  It also defines the PCI configuration
register identifier for PCI BAR0.

This patch also adds wrappers for 32-bit 'in' and 'out' port I/O
instructions.  They were placed in helpers.S, since they may be useful
to other modules besides just the PCI support module.

cpu/x86/helpers.S

@@ -43,8 +43,21 @@ outb:
         out %al, %dx
         ret
 
+.global outl
+outl:
+        mov 4(%esp), %dx
+        mov 8(%esp), %eax
+        out %eax, %dx
+        ret
+
 .global inb
 inb:
         mov 4(%esp), %dx
         in %dx, %al
         ret
+
+.global inl
+inl:
+        mov 4(%esp), %dx
+        in %dx, %eax
+        ret