i2c: tegra: Move out all device-tree parsing into tegra_i2c_parse_dt()

Move out code related to device-tree parsing from the probe function into
tegra_i2c_parse_dt() in order to make code more consistent.

Reviewed-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>