refactor(st): move boot backup register management

This backup register used to pass boot information to BL33, has the
same mapping for ST platforms. Its management can then be moved to
common directory.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ic873f099c1f87c6ba2825b4946365ae6a9687798
5 files changed