usb: phy: tegra: Add clarifying comments about the shared registers

Tools like Coccinelle may erroneously recommend to use the
devm_platform_ioremap_resource() API for the registers mapping because
these tools are not aware about the implementation details of the driver.
Let's add a clarifying comments to the code, which should help to stop
future attempts to break the driver.
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20200202224259.29187-1-digetx@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/phy/phy-tegra-usb.c

@@ -969,6 +969,10 @@ static int utmi_phy_probe(struct tegra_usb_phy *tegra_phy,
 		return  -ENXIO;
 	}
 
+	/*
+	 * Note that UTMI pad registers are shared by all PHYs, therefore
+	 * devm_platform_ioremap_resource() can't be used here.
+	 */
 	tegra_phy->pad_regs = devm_ioremap(&pdev->dev, res->start,
 					   resource_size(res));
 	if (!tegra_phy->pad_regs) {
@@ -1087,6 +1091,10 @@ static int tegra_usb_phy_probe(struct platform_device *pdev)
 		return  -ENXIO;
 	}
 
+	/*
+	 * Note that PHY and USB controller are using shared registers,
+	 * therefore devm_platform_ioremap_resource() can't be used here.
+	 */
 	tegra_phy->regs = devm_ioremap(&pdev->dev, res->start,
 				       resource_size(res));
 	if (!tegra_phy->regs) {