cactus: Use UART2 instead of UART0

It isn't a good idea to share resources between different images. For
this reason, Cactus should use a different console than the TF and TFTF.
Future partitions should ideally use another different UART.

Change-Id: I8b61ca09b0e820bc6b9cdb400c8d55e2005a23bd
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>