Allwinner A20 power up pin?
There isn't a "power on" pin per-say, but as with many SoC's and MCUs, they have a "reset" pin which does much the same function.

As per the table on page 16 of the A20 Product Brief, the A20 does indeed have a reset pin.

Furthermore, if you refer to page 33, you can see the intended power on sequence. First you apply power, while holding the reset pin low (asserted). Once the power is stabilised, you can then release the chip from reset by pulling the pin high. This ensures that none of the logic ends up in a weird state while the power is ramping up.