cactus: Fix base address in linkerscript

This helps debugging as the elf file contains the base address of the
image so it doesn't have to be specified when loading the debug symbols.

Change-Id: Ie9dcbce3d1b3d24ff587bd9daa1839b948e98f99
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>