This command seems to work pretty reliably from the U-Boot prompt:
i2c dev 0;i2c mw 0x58 0x00 0x00;i2c mw 0x58 0x4a 0x20;i2c mw 0x58 0x4b 0x80;i2c mw 0x58 0x06 0x4;i2c mw 0x58 0x0e 0x60;
It places the PMIC in hibernate mode, and uses its one-second tick alarm to immediately wake back up again. By replacing the last write with
i2c mw 0x58 0x13 0x2;
the board will perform a full power cycle instead. I found the tick alarm sequence in this thread on Dialog’s support forums.