u-boot/tools/binman/test/21_image_pad.dts

/dts-v1/;

/ {
	#address-cells = <1>;
	#size-cells = <1>;

	binman {
		pad-byte = <0xff>;
		u-boot-spl {
		};

		u-boot {
			pos = <20>;
		};
	};
};