configs/stm32mp157a_dk1_defconfig: unbreak TF-A build with GCC >= 12

With the move to default to GCC 12 in commit e0091e4 (package/gcc:
switch to gcc 12.x as the default), TF-A now fails to build as a warning is
generated and it builds with -Werror:

  CC      plat/st/stm32mp1/bl2_plat_setup.c
drivers/st/io/io_stm32image.c: In function ‘stm32image_partition_read’:
drivers/st/io/io_stm32image.c:249:13: error: ‘result’ may be used uninitialized [-Werror=maybe-uninitialized]
  249 |         int result;
      |             ^~~~~~
cc1: all warnings being treated as errors

This is fixed in TF-A v2.6 with commit c1d732d0db24 (fix(io_stm32image):
uninitialized variable warning), but I do not have the board to verify if
v2.6 works, so instead disable -Werror by passsing E=0.

Signed-off-by: Peter Korsgaard <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>

configs/stm32mp157a_dk1_defconfig

@@ -30,7 +30,7 @@ BR2_TARGET_ARM_TRUSTED_FIRMWARE=y
 BR2_TARGET_ARM_TRUSTED_FIRMWARE_CUSTOM_VERSION=y
 BR2_TARGET_ARM_TRUSTED_FIRMWARE_CUSTOM_VERSION_VALUE="v2.5"
 BR2_TARGET_ARM_TRUSTED_FIRMWARE_PLATFORM="stm32mp1"
-BR2_TARGET_ARM_TRUSTED_FIRMWARE_ADDITIONAL_VARIABLES="STM32MP_SDMMC=1 AARCH32_SP=sp_min DTB_FILE_NAME=stm32mp157a-dk1.dtb"
+BR2_TARGET_ARM_TRUSTED_FIRMWARE_ADDITIONAL_VARIABLES="STM32MP_SDMMC=1 AARCH32_SP=sp_min DTB_FILE_NAME=stm32mp157a-dk1.dtb E=0"
 BR2_TARGET_ARM_TRUSTED_FIRMWARE_IMAGES="*.stm32"
 BR2_TARGET_ARM_TRUSTED_FIRMWARE_NEEDS_DTC=y
 BR2_TARGET_UBOOT=y