dt-bindings: serial: Convert slave-device bindings to json-schema

Convert the serial slave-device Device Tree binding documentation to
json-schema, and incorporate it into the generic serial bindings.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Rob Herring <robh@kernel.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index b4cb364..16760fc 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -15001,7 +15001,7 @@
 M:	Rob Herring <robh@kernel.org>
 L:	linux-serial@vger.kernel.org
 S:	Maintained
-F:	Documentation/devicetree/bindings/serial/slave-device.txt
+F:	Documentation/devicetree/bindings/serial/serial.yaml
 F:	drivers/tty/serdev/
 F:	include/linux/serdev.h