interconnect: icc-rpm: Support child NoC device probe

As shown in downstream DT[1], the System NoC of QCM2290 is modelled
using 4 fab/noc devices: sys_noc + qup_virt + mmnrt_virt + mmrt_virt.
Among those 3 virtual devices, qup is owned by RPM and has no regmap
resource, while mmnrt and mmrt are owned by AP and share the same
regmap as sys_noc.  So it's logical to represent these virtual devices
as child nodes of sys_noc in DT, so that such configuration can be
supported with a couple of changes on qnoc_probe():

- If there are child nodes, populate them.
- If the device descriptor has .regmap_cfg but there is no IOMEM
  resource for the device, use parent's regmap.

[1] https://android.googlesource.com/kernel/msm-extra/devicetree/+/refs/tags/android-11.0.0_r0.56/qcom/scuba-bus.dtsiSigned-off-by: Shawn Guo <shawn.guo@linaro.org>
Link: https://lore.kernel.org/r/20211215002324.1727-4-shawn.guo@linaro.orgSigned-off-by: Georgi Djakov <djakov@kernel.org>

drivers/interconnect/qcom/icc-rpm.c

@@ -349,8 +349,13 @@ int qnoc_probe(struct platform_device *pdev)
 		void __iomem *mmio;
 
 		res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
-		if (!res)
+		if (!res) {
+			/* Try parent's regmap */
+			qp->regmap = dev_get_regmap(dev->parent, NULL);
+			if (qp->regmap)
+				goto regmap_done;
 			return -ENODEV;
+		}
 
 		mmio = devm_ioremap_resource(dev, res);
 
@@ -366,6 +371,7 @@ int qnoc_probe(struct platform_device *pdev)
 		}
 	}
 
+regmap_done:
 	ret = devm_clk_bulk_get(dev, qp->num_clks, qp->bus_clks);
 	if (ret)
 		return ret;
@@ -417,6 +423,10 @@ int qnoc_probe(struct platform_device *pdev)
 
 	platform_set_drvdata(pdev, qp);
 
+	/* Populate child NoC devices if any */
+	if (of_get_child_count(dev->of_node) > 0)
+		return of_platform_populate(dev->of_node, NULL, NULL, dev);
+
 	return 0;
 err:
 	icc_nodes_remove(provider);