dt-bindings: pinctrl: renesas,rza2-pinctrl: Fix pin controller node name

According to Devicetree Specification v0.2 and later, Section "Generic
Names Recommendation", the node name for a pin controller device node
should be "pinctrl".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200821111127.3771-1-geert+renesas@glider.be