pci: add CONFIG_PCI_ENUM_ONLY for platforms that don't need PCI setup done

Introduce CONFIG_PCI_ENUM_ONLY variable for platforms that just want a
quick enumberation of the PCI devices, but don't need any setup work done.
This is very beneficial on platforms that have u-boot loaded by another
boot loader which does a more sophisticated job of setup of PCI devices
than u-boot.  That way, u-boot can just read what's there and get on
with life.  This is what SeaBIOS does.

Signed-off-by: Andrew Sharp <andywyse6@gmail.com>
diff --git a/README b/README
index da4341f..d45eba7 100644
--- a/README
+++ b/README
@@ -3377,6 +3377,13 @@
 		Disable PCI-Express on systems where it is supported but not
 		required.
 
+- CONFIG_PCI_ENUM_ONLY
+		Only scan through and get the devices on the busses.
+		Don't do any setup work, presumably because someone or
+		something has already done it, and we don't need to do it
+		a second time.	Useful for platforms that are pre-booted
+		by coreboot or similar.
+
 - CONFIG_SYS_SRIO:
 		Chip has SRIO or not