support/testing: test_atf: bump the custom version to v2.5

This version bump is needed to pass the ATF test with
hardening option enabled (-fstack-protector-strong)

With the version v2.2, ATF fail due to undefined references:

./build/juno/release/bl2u/arm_tzc400.o: In function `arm_tzc400_setup':
arm_tzc400.c:(.text.arm_tzc400_setup+0x10): undefined reference to `__stack_chk_guard'
arm_tzc400.c:(.text.arm_tzc400_setup+0x18): undefined reference to `__stack_chk_guard'
arm_tzc400.c:(.text.arm_tzc400_setup+0xb8): undefined reference to `__stack_chk_guard'
arm_tzc400.c:(.text.arm_tzc400_setup+0xcc): undefined reference to `__stack_chk_fail'

Since commit ccac9a5b, Buildroot no
longer forces ENABLE_STACK_PROTECTOR. However, we rely on the ATF build
system to handle it correctly, and this wasn't the case in v2.2.

Fixes: https://gitlab.com/buildroot.org/buildroot/-/jobs/1524842591



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>