ram: stm32mp1: update BIST config for tuning

Update the BIST config to compute the real use mask for the real
bank, row and col of the used DDR. The values are get from addrmap
register value.

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