allwinner: regulators: pick correct DT subnode

So far the DT node describing the AXP803 PMIC used in many Allwinner A64
boards had only one subnode, so our code just entering the first subnode
to find all regulators worked fine.

However recent DT updates in the Linux kernel add more subnodes *before*
that, so we need to make sure to explicitly enter the "regulators"
subnode to find the information we are after.

Improve some DT node parsing error handling on the way.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

plat/allwinner/sun50i_a64/sunxi_power.c

@@ -229,8 +229,8 @@ static void setup_axp803_rails(const void *fdt)
 
 	/* locate the PMIC DT node, bail out if not found */
 	node = fdt_node_offset_by_compatible(fdt, -1, "x-powers,axp803");
-	if (node == -FDT_ERR_NOTFOUND) {
-		WARN("BL31: PMIC: No AXP803 DT node, skipping initial setup.\n");
+	if (node < 0) {
+		WARN("BL31: PMIC: Cannot find AXP803 DT node, skipping initial setup.\n");
 		return;
 	}
 
@@ -241,11 +241,15 @@ static void setup_axp803_rails(const void *fdt)
 	}
 
 	/* descend into the "regulators" subnode */
-	node = fdt_first_subnode(fdt, node);
+	node = fdt_subnode_offset(fdt, node, "regulators");
+	if (node < 0) {
+		WARN("BL31: PMIC: Cannot find regulators subnode, skipping initial setup.\n");
+		return;
+	}
 
 	/* iterate over all regulators to find used ones */
 	for (node = fdt_first_subnode(fdt, node);
-	     node != -FDT_ERR_NOTFOUND;
+	     node >= 0;
 	     node = fdt_next_subnode(fdt, node)) {
 		const struct axp_regulator *reg;
 		const char *name;