fvp: use two instances of Cactus at S-EL1
fvp: use two instances of Cactus at S-EL1

To demonstrate communication between SP's two instances of Cactus at
S-EL1 has been used.
This patch replaces Ivy SP with cactus-secondary SP which aligns with
changes in tf-a-tests repository.

Signed-off-by: Manish Pandey <f72a68396f72e54a6338d46190da5e42d7475246@arm.com>
Change-Id: Iee84f1f7f023b7c4f23fbc13682a42614a7f3707

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware