cmake: Fix debugger unit variable

When trying to build a platform with support for the CLI debugger,
the cmake build system will spit the following error:

CMake Warning:
  Manually-specified variables were not used by the project:

    SCP_ENABLE_DEBUG_INIT

This is caused by a typo in the variable name:
DSCP_ENABLE_DEBUG_INIT -> DSCP_ENABLE_DEBUGGER_INIT

This commit replaced the variable name used for the debug unit support
with the correct name for the CLI debugger.

Change-Id: Iae97edbebd9990535268b6eec025cf5aaa6f8c35
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>