ARM: OMAP4470: VC: update voltage levels for operating modes

Update retention voltage from:
Operating Conditions Data Manual Addendum
Revision 0.7 (October 2012).

Change-Id: Ia22ee35bf1f6d1e50c4f1d661a4e103d10fc3eda
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>