dt-bindings: bus: document ETZPC

Document ETZPC (Extended TrustZone protection controller). ETZPC is a
firewall controller.

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>