dt-bindings: treewide: add access-controllers description

access-controllers is an optional property that allows a peripheral to
refer to one or more domain access controller(s).

Description of this property is added to all peripheral binding files of
the peripheral under the STM32 firewall controller. It allows an accurate
representation of the hardware, where various peripherals are connected
to a firewall bus. The firewall can then check the peripheral accesses
before allowing its device to probe.

Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>