boot/arm-trusted-firmware: change ENABLE_STACK_PROTECTOR value when disabled

Since commit
cf176128 ("boot/arm-trusted-firmware:
add SSP option"), we are passing ENABLE_STACK_PROTECTOR=none when we
want to disable SSP usage in TF-A. While this works fine in recent
versions of TF-A, older versions such as TF-A will end up passing
-fstack-protector-none in this situation, which fails as this is not a
valid gcc option (the valid gcc option is -fno-stack-protector).

To solve this, we pass ENABLE_STACK_PROTECTOR=0 which was in older
TF-A versions used to say "don't do anything with SSP", and is also
still supported in newer versions of TF-A.

Fixes:

 https://gitlab.com/buildroot.org/buildroot/-/jobs/1478738580



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