Platform: Fix missing heap for GCC when TFM_LVL!=1

The heap section was missing when TF-M was built with GCC in level 2 or
3 with the secure client test partition disabled. It is required by the
GCC libc implementation of printf.

Change-Id: Id4cea624da783cad9a486e85d213093f20a58d70
Signed-off-by: Jamie Fox <jamie.fox@arm.com>