xtensa: ISS: add dummy serial.h for ISS platform

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>
diff --git a/arch/xtensa/platforms/iss/include/platform/serial.h b/arch/xtensa/platforms/iss/include/platform/serial.h
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/arch/xtensa/platforms/iss/include/platform/serial.h