dt-bindings: pinctrl: document the QCS615 Top Level Mode Multiplexer

Document the Top Level Mode Multiplexer on the QCS615 Platform.
It concisely explains the pin multiplexing and configuration in
the device tree, and includes simple examples of typical device
tree snippets, making it easier for designers to configure and
manage chip pins.

Signed-off-by: Lijuan Gao <quic_lijuang@quicinc.com>
Link: https://lore.kernel.org/20240920-add_qcs615_pinctrl_driver-v2-1-e03c42a9d055@quicinc.com


Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>