usb: phy: tegra: Add clarifying comments about the shared registers

Tools like Coccinelle may erroneously recommend to use the
devm_platform_ioremap_resource() API for the registers mapping because
these tools are not aware about the implementation details of the driver.
Let's add a clarifying comments to the code, which should help to stop
future attempts to break the driver.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20200202224259.29187-1-digetx@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>