pinctrl: tegra: xusb: Add note about legacy status

Add a comment about why the call to of_match_node() cannot be replaced
by of_device_get_match_data(). This will hopefully prevent people from
attempting to clean this up in the future.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20190923095400.GA11084@ulmo


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>