As described in section 7.10.2.2.2 in the data sheet, all power rails except VDD_CORE and VDDR_CORE can be ramped down independent of each other. If VDD_CORE is operating at 0.75 V, it should be ramped down after the 0.85 V VDDR_CORE. For the case where VDD_CORE and VDDR_CORE both run from a common 0.85 V supply, there is no ramp down requirement between them.