boards: arm: mps2: Add dependency to empty_cpu0 for an521/cpu1's app

`west build` does not build the `empty_cpu0` application making
`west build -b mps2/an521/cpu1 -t run` fail because of the missing
empty_cpu0's binary.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>
(cherry picked from commit 52151d0b)