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/arch/arm/Kconfig b/arch/arm/Kconfig
index bee626f..0c1b754 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1755,6 +1755,7 @@
 	select XEN
 	select OF_CONTROL
 	select LINUX_KERNEL_IMAGE_HEADER
+	select XEN_SERIAL
 endchoice
 
 config ARCH_SUPPORT_TFABOOT