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>