cactus: generate JSON file required by TF-A

Generate JSON file which is being passed as "SP_LAYOUT_FILE" build
parameter to TF-A build.
Two instances of cactus is used to demonstrate functioning of multiple
SP's. cactus-secondary dts is same as cactus dts except for UUID and
load address.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ib6d8f7d5d4894495754da0ce39f663263eeb182f