board/stmicroelectronics/stm32mp157-dk: rename uboot partition in genimage.cfg

Change partition name according to the U-Boot requirements.

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/stmicroelectronics/stm32mp157-dk/genimage.cfg

@@ -11,7 +11,7 @@ image sdcard.img {
 		image = "u-boot-spl.stm32"
 	}
 
-	partition uboot {
+	partition ssbl {
 		image = "u-boot.img"
 	}