stm32mp: fdt: update etzpc for STM32MP15x

Introduce STM32MP15 function and defines to prepare the
STM32MP13 introduction.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Change-Id: I909b205e73dcf207e0216aae5905c3c52472020e