These TEGRA_IO_RAIL_... functions and constants have been deprecated in
commit 21b49910 ("soc/tegra: pmc: Add I/O pad voltage support") in
2016-11.

There seems to be no users since kernel 4.16.

Remove them now.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Thierry Reding <treding@nvidia.com>