gpio: tegra: Use generic readl_relaxed/writel_relaxed accessors

There is no point in using old-style raw accessors, the generic accessors
do the same thing and also take into account CPU endianness. Tegra SoCs do
not support big-endian mode in the upstream kernel, but let's switch away
from the outdated things anyway, just to keep code up-to-date.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>