[yocto] Power off IC over Yocto

[simo]

I am using a Gumtix Overo board and we create and expansion board that
power Overo and other devices by external power supply IC.

Our expansion board with power supply are connected by I2C bus 2 in
overo (not the same i2C that TW4030 or tps6595 is connected).

Using the Linux  command shutdown we would like to power off the hole
system.
We would like to close the linux and last thing send a command over I2C
to our power supply IC in order it disconnect the physical power.

Our question is:

Where we have to implement that function?
Is there any example of it?