documentation: Fix pinctrl documentation for Meson8 / Meson8b

Fix pin controller documentation introducing the new compatibles for
the pinctrl drivers specific for aobus / cbus.

This is needed because we have changed the pin controller driver: we
have now a single specialized pinctrl driver / compatible for each bus
the controller is attached to, instead of one single driver dealing with
all the controllers we have on different buses.

Signed-off-by: Carlo Caione <carlo@endlessm.com>
Acked-by: Rob Herring <robh@kernel.org>