pinctrl: bcm281xx: Add support for BCM21664 pinmux

BCM21664 is another chip from the Kona line of Broadcom SoCs, and
its pinmux shares a lot of similarities with the BCM281xx pinmux.

Add support for the BCM21664 pinmux controller to the BCM281xx driver.

This also enables pinmux support for the BCM23550, which has an
identical pinmux config to the BCM21664 (hence they can share a
single compatible, brcm,bcm21664-pinctrl).

Signed-off-by: Artur Weber <aweber.kernel@gmail.com>
Link: https://lore.kernel.org/20250303-bcm21664-pinctrl-v3-4-5f8b80e4ab51@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>