stm32mp2: Fix CONFIG_STM32MP25X flag usage

"#if" was used instead of "#ifdef"

Fixes: 01a70199 ("stm32mp2: initial support")
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>