ARM: dts: iwg20d-q7-common: Fix backlight pwm specifier

make dtbs_check:

    arch/arm/boot/dts/renesas/r8a7743-iwg20d-q7.dtb: backlight: pwms: [[58, 0, 5000000], [0]] is too long
	    From schema: Documentation/devicetree/bindings/leds/backlight/pwm-backlight.yaml
    arch/arm/boot/dts/renesas/r8a7743-iwg20d-q7-dbcm-ca.dtb: backlight: pwms: [[67, 0, 5000000], [0]] is too long
	    From schema: Documentation/devicetree/bindings/leds/backlight/pwm-backlight.yaml
    arch/arm/boot/dts/renesas/r8a7744-iwg20d-q7-dbcm-ca.dtb: backlight: pwms: [[67, 0, 5000000], [0]] is too long
	    From schema: Documentation/devicetree/bindings/leds/backlight/pwm-backlight.yaml
    arch/arm/boot/dts/renesas/r8a7744-iwg20d-q7.dtb: backlight: pwms: [[58, 0, 5000000], [0]] is too long
	    From schema: Documentation/devicetree/bindings/leds/backlight/pwm-backlight.yaml

PWM specifiers referring to R-Car PWM Timer Controllers should contain
only two cells.

Fix this by dropping the bogus third cell.

Fixes: 6f89dd9e ("ARM: dts: iwg20d-q7-common: Add LCD support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/6e5c3167424a43faf8c1fa68d9667b3d87dc86d8.1684855911.git.geert+renesas@glider.be