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>