Starting with Tegra186, the XUSB controller no longer has the IPFS
wrapper. This commit adds a "has_ipfs" field to struct tegra_xusb_soc
that can be used to declare the existence of the IPFS wrapper.
For the existing chips (i.e. Tegra124 and Tegra210), the new field is
set to true. A future patch adding support for Tegra186 will set it to
false.