dt-bindings: mfd: Add vref_ddr supply for STPMIC1

Add vref_ddr supply description for the STPMIC1.

Signed-off-by: Pascal Paillet <p.paillet@foss.st.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230523074206.201962-1-p.paillet@foss.st.com


Signed-off-by: Lee Jones <lee@kernel.org>