core/arm32: set L2 prefetch offset to 7

Change-Id: I4c1c4911c6694d1882079fec78c6f710d66f7145
Reviewed-on: https://gerrit.st.com/17052
Reviewed-by: Etienne CARRIERE <etienne.carriere@st.com>
Tested-by: Etienne CARRIERE <etienne.carriere@st.com>
Reviewed-by: Pascal BRAND <pascal.brand@st.com>