core: dt_driver: allow parsing of phandle == 0

In Linux, it is allowed to specify a null phandle which means it should
be skipped. Add support for this specific case by simply skipping over
it. This is needed to parse assigned-clock-parents which can use such
syntax. This is specified in the clock bindings [1] which says the
following:

  To skip setting parent or rate of a clock its corresponding entry
  should be set to 0, or can be omitted if it is not followed by any
  non-zero entry

For example this is a valid device-tree description:

assigned-clocks = <foo>, <bar>;
assigned-clock-parents = <0> <bar_parent>;
assigned-clock-rates = <1000>;

Link: [1] https://www.kernel.org/doc/Documentation/devicetree/bindings/clock/clock-bindings.txt
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Clément Léger <clement.leger@bootlin.com>

core/kernel/dt_driver.c

@@ -266,6 +266,13 @@ void *dt_driver_device_from_node_idx_prop(const char *prop_name,
 	while (idx < len) {
 		idx32 = idx / sizeof(uint32_t);
 		phandle = fdt32_to_cpu(prop[idx32]);
+		if (!phandle) {
+			if (!prop_idx)
+				break;
+			idx += sizeof(phandle);
+			prop_idx--;
+			continue;
+		}
 
 		prv = dt_driver_get_provider_by_phandle(phandle, type);
 		if (!prv) {