dt-bindings: arm-boards: Update pointer to ARM CPU bindings

The ARM CPU DT bindings were converted from plain text to YAML, but not
all referrers were updated.

Fixes: 672951cb ("dt-bindings: arm: Convert cpu binding to json-schema")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>