Using GIC_CPU_MASK_SIMPLE(x), x should reflect the number of CPUs.
STM32MP251 is a single core Cortex A35, STM32MP253 is a dual core CA35.

Fixes: 5d30d03a ("arm64: dts: st: introduce stm32mp25 SoCs family")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>