lib: add GENERIC_PCI_IOMAP

Many architectures want a generic pci_iomap but
not the rest of iomap.c. Split that to a separate .c
file and add a new config symbol. select automatically
by GENERIC_IOMAP.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/lib/Kconfig b/lib/Kconfig
index 0058927..36884b4 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -19,8 +19,12 @@
 config GENERIC_FIND_FIRST_BIT
 	bool
 
+config GENERIC_PCI_IOMAP
+	bool
+
 config GENERIC_IOMAP
 	bool
+	select GENERIC_PCI_IOMAP
 
 config CRC_CCITT
 	tristate "CRC-CCITT functions"