dt-bindings: pinctrl: Add support for Broadcom STB pin controller

The STB pin controller represents a family whose silicon instances
are found e.g. on BCM2712 SoC.

In particular, on RaspberryPi 5, there are two separate instantiations
of the same IP block which differ in the number of pins that are
associated and the pinmux functions for each of those pins. The
-aon- variant stands for 'Always On'.

Depending on the revision of the BCM2712 (CO or D0), the pin
controller instance has slight differences in the register layout.

Signed-off-by: Ivan T. Ivanov <iivanov@suse.de>
Signed-off-by: Andrea della Porta <andrea.porta@suse.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
[linusw: Dropped extranous label and fixed whitespace]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>