Hiya,
I believe the P55 chipset uses ICH10R, so my guess would be it's software RAID (or a hybrid). I don't have personal experience getting ESX to detect one of these RAIDs, but I would be willing to bet you'd have an easier time if you downloaded/installed drivers that are linux-approved for the Matrix controller.
I have tried installing ESX on machines running P55 chipset boards before and I've had mixed results. (A lot of times other onboard features won't be supported or will crap out the installer, for ex. onboard NICs)