refactor(stm32mp1-fdts): update regulator description

Update regulator description to match with pmic driver updates.
vref_ddr does not support over-current protection.
vtt_ddr is set to sink source mode.

Change-Id: I725f35b091ca8c230994c2b5f81693ebc97bf4aa
Signed-off-by: Pascal Paillet <p.paillet@st.com>
Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>