dt-bindings: rtc: rx8900: Convert to YAML schema

Convert the Epson RX8900 DT bindings to YAML schema.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: kernel@dh-electronics.com
Cc: linux-rtc@vger.kernel.org
To: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210610150044.29792-1-marex@denx.de