If BL2_AT_EL3 is enabled, bl2_run_next_image is called at the end of BL2.
This function calls disable_mmu_icache_secure.
It is then useless to call it in bl2_main in that case.

fixes arm-software/tf-issues#582

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