hardening: Require CONFIG_TIMER_RANDOM_GENERATOR=n

This commit adds the requirement for the `TIMER_RANDOM_GENERATOR`
Kconfig to be set to `n` because it enables timer-based pseudo-random
number generator, which is only intended to be used for testing
purposes.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>