serial: add pci variant

So we get a hot-pluggable 16550 uart.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/default-configs/pci.mak b/default-configs/pci.mak
index 69e18f1..ae9d1eb 100644
--- a/default-configs/pci.mak
+++ b/default-configs/pci.mak
@@ -19,3 +19,5 @@
 CONFIG_AHCI=y
 CONFIG_ESP=y
 CONFIG_ESP_PCI=y
+CONFIG_SERIAL=y
+CONFIG_SERIAL_PCI=y