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/Makefile b/lib/Makefile
index a4da283..609b2adc 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -33,6 +33,7 @@
 
 lib-$(CONFIG_HOTPLUG) += kobject_uevent.o
 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
+obj-$(CONFIG_GENERIC_PCI_IOMAP) += pci_iomap.o
 obj-$(CONFIG_HAS_IOMEM) += iomap_copy.o devres.o
 obj-$(CONFIG_CHECK_SIGNATURE) += check_signature.o
 obj-$(CONFIG_DEBUG_LOCKING_API_SELFTESTS) += locking-selftest.o