soc/tegra: pmc: Use devm_clk_get_optional()

Use devm_clk_get_optional() instead of hand writing it.

While at it, use dev_err_probe() to further simplify the code. This is also
less verbose if clk_get() returns -EPROBE_DEFER.

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