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At each BL entry point, the registers r9 to r12 are used to save info from
the previous BL parameters put in r0 to r3. But zeromem uses r12, leading
to a corruption of arg3. Therefore this change copies r12 to r7 before
zeromem() call and restores r12 afterwards. It may be better to save it
in r7 in el3_arch_init_common and not at the entrypoint as r7 could be used
in other functions, especially platform ones.
This is a fix for Task T661.

Change-Id: Icc11990c69b5d4c542d08aca1a77b1f754b61a53
Signed-off-by: Yann Gautier <yann.gautier@st.com>