configs/stm32mp157c_dk2_defconfig: update arm-trusted-firmware to lts-v2.8.13

The assumption is that updates to the stable branch should not break anything,
so we use the latest LTS rather than a specific version.

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

configs/stm32mp157c_dk2_defconfig

@@ -27,8 +27,7 @@ BR2_TARGET_ROOTFS_EXT2_SIZE="120M"
 
 # Bootloaders
 BR2_TARGET_ARM_TRUSTED_FIRMWARE=y
-BR2_TARGET_ARM_TRUSTED_FIRMWARE_CUSTOM_VERSION=y
-BR2_TARGET_ARM_TRUSTED_FIRMWARE_CUSTOM_VERSION_VALUE="v2.6"
+BR2_TARGET_ARM_TRUSTED_FIRMWARE_LATEST_LTS_2_8_VERSION=y
 BR2_TARGET_ARM_TRUSTED_FIRMWARE_PLATFORM="stm32mp1"
 BR2_TARGET_ARM_TRUSTED_FIRMWARE_FIP=y
 BR2_TARGET_ARM_TRUSTED_FIRMWARE_BL31=y