dt-bindings: serial: Move renesas,rzn1-uart into the snps-dw-apb-uart binding

The renesas,rzn1-uart binding only differs in compatible string from the
snps-dw-apb-uart binding. Move it there, converting it to json-schema in
the process.

Cc: Phil Edworthy <phil.edworthy@renesas.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-serial@vger.kernel.org
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>