cassini-bsp: Keep wic.pad image to fix local Corstone-1000 FVP usage

The Corstone-1000 FVP config refers to the uncompressed rootfs image
(.rootfs.wic.pad) however the build only deploys a compressed image
(.rootfs.wic.pad.gz). When following the instructions in the user guide
the Corstone-1000 FVP therefore fails to find the Cassini rootfs image
and falls back to the Corstone-1000 flash image.

Explicitly add wic.pad to IMAGE_FSTYPES for the FVP so that the
instructions in the user guide behave as expected.

This is not a problem for CI as the compressed image is extracted
automatically by LAVA.

Changelog: bug
Signed-off-by: Adam Johnston <adam.johnston@arm.com>