ddr: stm32mp1_ddr: correctly PWR DDR retention bits

If the DDR is not in self-refresh, the PWR DDR retention bits should be
cleared, whatever the status of PWR_CR3_DDRRETEN. This can happen in case
of STANDBY with DDR of, if the DDR has not been set in Self Refresh.

Change-Id: If8854b06ad4d2a8d0bf6e227c411efcdcd4b004a
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/188935


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Sebastien PASDELOUP <sebastien.pasdeloup-ext@st.com>