make dtbs_check W=2:

    arch/arm/boot/dts/renesas/r8a7790-lager.dts:444.11-458.5: Warning (interrupt_provider): /i2c-mux4/pmic@58: Missing '#interrupt-cells' in interrupt provider
    ...

Fix this by adding the missing #interrupt-cells properties.
Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/a351e503ea97fb1af68395843f513925ff1bdf26.1707922460.git.geert+renesas@glider.be