arm-bsp-extras/scp-firmware/fvp-rd-aspen: Enable debugger CLI

The RD-Aspen FMU and SSU integration tests will use the SCP-Firmware
debugger CLI and debugger module, which has to be explicitly enabled.

Enable this parameter for all builds so that the integration tests are
accessible.

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>