dt-bindings: i2c: renesas,riic: Add interrupt-names

The Renesas RZ/A and RZ/G2L I2C Bus Interface has no less than 8
interrupts.  Hence document the "interrupt-names" property, to make it
easier to review the interrupt mappings in DTS files.

Note that this property cannot be made required yet, as the RIIC nodes
in all DTS files lack the property.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/a81d0e14e395f297666e8c3a8ce3e292d2606a65.1626267422.git.geert+renesas@glider.be