Allow to configure the retram state in standby-ddr-sr from
the device-tree. This is realized by introducing a new binding
property to stm32mp1 pwr node: st,retram-enabled-in-standby-ddr-sr.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Change-Id: Iacd15321bce06e071a82bdfa05e4b63c3bf8c4fa