plat-stm: set L2 prefetch offset to 7
plat-stm: set L2 prefetch offset to 7

Reviewed-on: https://gerrit.st.com/17052
Reviewed-by: Etienne CARRIERE <f21bc5b8f3ea1962eb45af359f1abea6578cab94@st.com>
Tested-by: Etienne CARRIERE <f21bc5b8f3ea1962eb45af359f1abea6578cab94@st.com>
Reviewed-by: Pascal BRAND <0b55292d2562685bad4ce52d64f802e8f1cd299e@st.com>
Reviewed-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>
Signed-off-by: Pascal Brand <0b55292d2562685bad4ce52d64f802e8f1cd299e@st.com>

jamescfkung/optee_os,pascal-brand-st-dev/optee_os,BreezeWu/optee_os,chshxi1989/optee_os,jamescfkung/optee_os,BreezeWu/optee_os,chshxi1989/optee_os,matt2048/optee_os,BreezeWu/optee_os,Microsoft/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,BreezeWu/optee_os,matt2048/optee_os,Microsoft/optee_os,chshxi1989/optee_os,chshxi1989/optee_os,pascal-brand-st-dev/optee_os,BreezeWu/optee_os,cedric-chaumont-st-dev/optee_os,pascal-brand-st-dev/optee_os,cedric-chaumont-st-dev/optee_os,jamescfkung/optee_os,Microsoft/optee_os,chshxi1989/optee_os,Microsoft/optee_os,jamescfkung/optee_os,cedric-chaumont-st-dev/optee_os,matt2048/optee_os,jamescfkung/optee_os,matt2048/optee_os,cedric-chaumont-st-dev/optee_os,Microsoft/optee_os