stm32mp: update dependency for STM32_ETZPC

Correct the dependency for STM32 ETZPC protection, linked to SOC
STM32MP identified by CONFIG_STM32MP15x and not linked to
CONFIG_TARGET_STM32MP1 (no more existing).

This patch fix an issue introduced by commit 84625488 ("stm32mp1:
split board and SOC support for STM32MP15x family").

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>