ARM: dts: stm32: add ETZPC as a system bus for STM32MP1x boards

ARM: dts: stm32: add ETZPC as a system bus for STM32MP1x boards

The STM32 System Bus is an internal bus on which devices are connected.
ETZPC is a peripheral overseeing the firewall bus that configures
and control access to the peripherals connected on it.

For more information on which peripheral is securable, please read
the STM32MP13 or STM32MP15 reference manual.

Signed-off-by: Lionel Debieve <lionel.debieve@foss.st.com>
Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>