serial: serial_xen: Add Xen PV serial driver

Add support for Xen para-virtualized serial driver. This
driver fully supports serial console for the virtual machine.

Please note that as the driver is initialized late, so no banner
nor memory size is visible.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/xenguest_arm64_defconfig b/configs/xenguest_arm64_defconfig
index f730799..45559a1 100644
--- a/configs/xenguest_arm64_defconfig
+++ b/configs/xenguest_arm64_defconfig
@@ -49,7 +49,7 @@
 
 CONFIG_DM=y
 # CONFIG_MMC is not set
-# CONFIG_DM_SERIAL is not set
+CONFIG_DM_SERIAL=y
 # CONFIG_REQUIRE_SERIAL_CONSOLE is not set
 
 CONFIG_OF_BOARD=y