dt-bindings: i2c: snps,designware-i2c: describe Renesas RZ/N1D variant

So far, no differences are known, so it can fallback to the default
compatible.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>