arm64: dts: salvator-x: Add missing index to PWM pinctrl subnode name

R-Car Gen3 SoCs contain multiple PWM modules.  Hence to avoid conflicts,
pinctrl subnodes for PWM should include indices referring to their
instances.

Fixes: b33be336 ("arm64: dts: salvator-x: Add panel backlight support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>