stm32mp1: add a function to get non-secure DDR size

This function gets the DDR size from DT, and withdraws (if defined) the
sizes of secure DDR and shared memory areas.
This function also checks DT values fits the default DDR range.
This non-secure memory is available for BL33 and non-secure OS.

Change-Id: I162ae5e990a0f9b6b7d07e539de029f1d61a391b
Signed-off-by: Yann Gautier <yann.gautier@st.com>