binman: Allow sections to have an offset

At present sections are always placed automatically. Even if an 'offset'
property is provided it is ignored. Update the logic to support an offset
for sections.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

tools/binman/test/101_sections_offset.dts

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		pad-byte = <0x26>;
+		size = <0x38>;
+		section@0 {
+			read-only;
+			offset = <0x4>;
+			size = <0x10>;
+			pad-byte = <0x21>;
+
+			u-boot {
+			};
+		};
+		section@1 {
+			size = <0x10>;
+			pad-byte = <0x61>;
+			offset = <0x18>;
+
+			u-boot {
+			};
+		};
+		section@2 {
+			offset = <0x2c>;
+			u-boot {
+			};
+		};
+	};
+};