pinctrl: tegra: xusb: Silence sparse warnings

Commit 53d2a715 ("phy: Add Tegra XUSB pad controller support") added
a new driver for the XUSB pad controller that implements a more flexible
devicetree binding. In order to preserve backwards compatibility the old
driver can be probed if the obsolete bindings are detected.

In order to hide the legacy code, these prototypes were defined in a
header private to the new driver. This has the disadvantage of making
the sparse code checker complain about the missing declarations when
compiling the old driver and suggesting to make the functions static.

Avoid these sparse warnings by adding local prototype declarations into
the compatibility driver.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>