configs/stm32mp157*: bump ATF to v2.6

- v2.6 changes to FIP instead of SSBL boot.
- This requires switching to u-boot.bin in U-Boot, and using that
  instead of .stm32 in ATF.
- fiptool expects dtb to be split off from u-boot.bin, so need custom
  u-boot-nodtb.bin in addition to u-boot.dtb.
  Caveat: this is perhaps not really obligatory, but the ATF
          documentation explains it like that.
- The partition must be named fip (while it must be named ssbl for
  SSBL boot).
- Since the partition name is set in the common template, all
  defconfigs using that template must be bumped in a single commit
- ATF now produces both fip.bin and <boardname>.stm32, so fip.bin has
  been added to the configs
- While we're at it, add the structuring comments to
  avenger96_defconfig similar like the other defconfigs

Signed-off-by: Marleen Vos <marleen.vos@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/stmicroelectronics/common/stm32mp157/genimage.cfg.template

@@ -11,8 +11,8 @@ image sdcard.img {
 		image = "%ATFBIN%"
 	}
 
-	partition ssbl {
-		image = "u-boot.stm32"
+	partition fip {
+		image = "fip.bin"
 		size = 2M
 	}