First, find the right Intel Controller (this may be different on your machine):
root@macmini:~# lspci | grep LPC
00:1f.0 ISA bridge: Intel Corporation HM65 Express Chipset Family LPC Controller (rev 05)
Next, look up the datasheet for that controller. In this case, it's https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/6-chipset-c200-chipset-datasheet.pdf
Read it. I wanted the AFTERG3_EN bit in the "GEN_PMCON_3 - General PM Configuration 3 Register", which documented on pp.510-513.
Offset Address is A4h and Size: 16-bit.
