configs/arm_juno: bump ATF to v1.3

ATF in version 1.2 fails to build with:

./build/juno/release/bl1/context_mgmt.o: In function `cm_prepare_el3_exit':
context_mgmt.c:(.text.cm_prepare_el3_exit+0x54): undefined reference to `cm_set_next_context'
context_mgmt.c:(.text.cm_prepare_el3_exit+0x54): relocation truncated to fit: R_AARCH64_JUMP26 against undefined symbol `cm_set_next_context'

This has been fixed in ATF v1.3. Even though there are even newer
versions of ATF available, we take a conservative approach, and bump
to the first version that has the build issue fixed.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/88314771



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>