stm32mp1: check if the SoC is single core

stm32mp1: check if the SoC is single core

Among the variants of STM32MP, the STM32MP151 is a single Cortex-A7 chip.
A function is added to check the part number of the SoC.
If it corresponds to STM32MP151A or STM32MP151C, then the chip has a single
Cortex-A7.

Change-Id: Icac2015c5d03ce0bcb8e99bbaf1ec8ada34be49c
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>