soc/tegra: Add comment over devm_pm_opp_set_clkname()

Explain why special handling was required here, it isn't obvious at all.
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/soc/tegra/common.c

@@ -108,6 +108,13 @@ int devm_tegra_core_dev_init_opp_table(struct device *dev,
 	u32 hw_version;
 	int err;
 
+	/*
+	 * For some devices we don't have any OPP table in the DT, and in order
+	 * to use the same code path for all the devices, we create a dummy OPP
+	 * table for them via this call. The dummy OPP table is only capable of
+	 * doing clk_set_rate() on invocation of dev_pm_opp_set_rate() and
+	 * doesn't provide any other functionality.
+	 */
 	err = devm_pm_opp_set_clkname(dev, NULL);
 	if (err) {
 		dev_err(dev, "failed to set OPP clk: %d\n", err);