I am not a kernel guy but some reason the linux driver is not up to snuff as the BSD driver is. I suspect that since BSD is more inclined to accept binary only drivers that no one has written a good enough crypto interface driver for linux, but that’s just my bias opinion.
Linux has a cryptographic driver interface, its called…Crypto API. As stated, support for the Geode LX HW crypt was added in 2.6.20 via module geode-aes. The Linx crypto engine even supports the Hifn cards which are popular cards recommended for use on Soekris and ALIX boards with pfSense or m0n0wall.