highbank: scan into hb_sregs DT subnodes

The DT used for Calxeda Highbank and Midway systems exposes a "system
registers" block, modeled as a DT subnode.
This includes several clocks, including the two fixed clocks for the
main oscillator and timer.

So far U-Boot was ignorant of this special construct (a "clocks" node
within the "hb-sregs" node), as it didn't need the PLL clocks in there.
But that also meant we lost the fixed clocks, which form the base for
the UART baudrate generator and also the SP804 timer.

To allow the generic PL011 and SP804 driver to read the clock rate,
add a simple bus driver, which triggers the DT node discovery inside this
special node. As we only care about the fixed clocks (we don't have
drivers for the PLLs anyway), just ignore the address translation (for
now).

The binding is described in bindings/arm/calxeda/hb-sregs.yaml, the DT
snippet in question looks like:

=======================
	sregs@fff3c000 {
		compatible = "calxeda,hb-sregs";
		reg = <0xfff3c000 0x1000>;

		clocks {
			#address-cells = <1>;
			#size-cells = <0>;

			osc: oscillator {
				#clock-cells = <0>;
				compatible = "fixed-clock";
				clock-frequency = <33333000>;
			};
			....
		};
	};
=======================

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

board/highbank/Makefile

@@ -3,4 +3,4 @@
 # (C) Copyright 2000-2006
 # Wolfgang Denk, DENX Software Engineering, wd@denx.de.
 
-obj-y	:= highbank.o ahci.o
+obj-y	:= highbank.o ahci.o hb_sregs.o

board/highbank/hb_sregs.c

@@ -0,0 +1,45 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Calxeda Highbank/Midway "system registers" bus driver
+ *
+ * There is a "clocks" subnode inside the top node, which groups all clocks,
+ * both programmable PLLs as well as fixed clocks.
+ * Simple allow the DT enumeration to look inside this node, so that we can
+ * read the fixed clock frequencies using the DM clock framework.
+ *
+ * Copyright (C) 2019 Arm Ltd.
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <dm/lists.h>
+
+static int hb_sregs_scan_fdt_dev(struct udevice *dev)
+{
+	ofnode clock_node, node;
+
+	/* Search for subnode called "clocks". */
+	ofnode_for_each_subnode(clock_node, dev_ofnode(dev)) {
+		if (!ofnode_name_eq(clock_node, "clocks"))
+			continue;
+
+		/* Enumerate all nodes inside this "clocks" subnode. */
+		ofnode_for_each_subnode(node, clock_node)
+			lists_bind_fdt(dev, node, NULL, NULL, false);
+		return 0;
+	}
+
+	return -ENOENT;
+}
+
+static const struct udevice_id highbank_sreg_ids[] = {
+	{ .compatible = "calxeda,hb-sregs" },
+	{}
+};
+
+U_BOOT_DRIVER(hb_sregs) = {
+	.name = "hb-sregs",
+	.id = UCLASS_SIMPLE_BUS,
+	.bind = hb_sregs_scan_fdt_dev,
+	.of_match = highbank_sreg_ids,
+};