Recent systems with Thunderbolt ports may support IOMMU natively. In
practice this means that Thunderbolt connected devices are placed behind
an IOMMU during the whole time it is connected (including during boot)
making Thunderbolt security levels redundant. This is called Kernel DMA
protection [1] by Microsoft.

Some of these systems still have Thunderbolt security level set to
"user" in order to support OS downgrade (the older version of the OS
might not support IOMMU based DMA protection so connecting a device
still relies on user approval).

Export this information to userspace by introducing a new sysfs
attribute (iommu_dma_protection). Based on it userspace tools can make
more accurate decision whether or not authorize the connected device.

In addition update Thunderbolt documentation regarding IOMMU based DMA
protection.

[1] https://docs.microsoft.com/en-us/windows/security/information-protection/kernel-dma-protection-for-thunderboltSigned-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Yehezkel Bernat <YehezkelShB@gmail.com>