ddr: stm32mp1_ddr: correct SELFREF_TO_X32 mask

In DDR controller PWRTMG register, the mask for field SELFREF_TO_X32 is
wrong. This field is from bit 16 to 23.

Change-Id: Id336fb08c88f0a153df186dd819e41af72febb88
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/192783


Reviewed-by: CITOOLS <MDG-smet-aci-reviews@list.st.com>
Reviewed-by: CIBUILD <MDG-smet-aci-builds@list.st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@foss.st.com>
Tested-by: Yann GAUTIER <yann.gautier@foss.st.com>