TMDXIDK5718: How to change PRU MII IO Level from 3.3 to1.8V?

Seem RGMII can not work by 2.5V? Just 1.8V and 3.3V.

Is same as PRU MII?

The TMDXIDK5718 EVM only supports 3.3V IO for the PRU MII.  However - the processor (AM5718) supports either 1.8V or 3.3V on all of its LVCMOS IO, including those IO that support PRU MII.  The AM5718 devices does not support 2.5V IO.

I  can not find the MII Power Pin definition in the datasheet?

Do you know which one?

In the data manual, Table 4-2 (ball characteristics) list each ball, and the various functions supported on those pins.  The power column defines which IO power rails that pins is supported.  (Each ball support multiple functions, some including MII functions).

Also - IOSETs for PRU-ICSS MII interfaces are listed in Table 7-154, 155, and shows the associated ball location.