dt-bindings: serial: convert Cadence UART bindings to YAML

Convert serial for Cadence UART bindings documentation to YAML.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Link: https://lore.kernel.org/r/20210715092252.225740-1-iwamatsu@nigauri.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>