Define DDR Self Refresh (SR) mode read/set/save/restore API functions.
At the end of DDR initialization in BL2, switch SR mode to the one
indicated by DDR updated registers. Save mode at BL32 init.
When entering in cstop/cstandby, switch to SSR (Software).
When exiting, go back to the saved mode.

Change-Id: Iadd795d20fdea2d7be30c1ba6f02eb172e33f4b1
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>