powerpc/mpc85xx: socrates: rework build process

U-Boot build process for socrates board produces final U-Boot binary in
file u-boot-socrates.bin (by binman) And as a bonus it produces two
unusable broken binaries u-boot-dtb.bin and u-boot.bin (by Makefile).

Clean this up, so final U-Boot binary is in u-boot-dtb.bin

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/arch/powerpc/dts/u-boot.dtsi b/arch/powerpc/dts/u-boot.dtsi
index 6b7375c..c39ab6f 100644
--- a/arch/powerpc/dts/u-boot.dtsi
+++ b/arch/powerpc/dts/u-boot.dtsi
@@ -9,7 +9,7 @@
 
 / {
 	binman {
-		filename = "u-boot.bin";
+		filename = "u-boot-dtb.bin";
 		skip-at-start = <CONFIG_TEXT_BASE>;
 		sort-by-offset;
 		pad-byte = <0xff>;