dt: serial: Add Renesas RZ/N1 binding documentation

The RZ/N1 UART is a modified Synopsys DesignWare UART.
The modifications only relate to DMA so you could actually use the
controller with the Synopsys compatible string if you are not using
DMA, but you should not do so.
Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/renesas,rzn1-uart.txt

+Renesas RZ/N1 UART
+
+This controller is based on the Synopsys DesignWare ABP UART and inherits all
+properties defined in snps-dw-apb-uart.txt except for the compatible property.
+
+Required properties:
+- compatible : The device specific string followed by the generic RZ/N1 string.
+   Therefore it must be one of:
+   "renesas,r9a06g032-uart", "renesas,rzn1-uart"
+   "renesas,r9a06g033-uart", "renesas,rzn1-uart"