dt-bindings: i2c: renesas,riic: Make interrupt-names required

Now the I2C device nodes in all DTS files have gained "interrupt-names"
properties, the "interrupt-names" property can be made required.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/da8d1973dcd419d8d9c8c662ee614952f3a6969e.1626267422.git.geert+renesas@glider.be