dt-bindings: pinctrl: Add bindings for BCM21664 pin controller

dt-bindings: pinctrl: Add bindings for BCM21664 pin controller

Add device tree bindings for the pin controller included in the
BCM21664 chip. The bindings are based off brcm,bcm11351-pinctrl.yaml;
both chips use the same driver, but have different pins, and the
BCM21664 has 6 alt modes instead of 4.

Signed-off-by: Artur Weber <aweber.kernel@gmail.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/20250303-bcm21664-pinctrl-v3-1-5f8b80e4ab51@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>