stm32mp: simplify the STM32MP15x package parsing code

Simplify the package parsing code for STM32MP15X as package can be
affected with get_cpu_package() result.

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