fwk: Extend delayed response support

Add delayed response support when !BUILD_HAS_MULTITHREADING is not
defined. This will enable all build modes to utilize deferred
responses.

Change-Id: I2c3d6b020ae3a5b9af605c18d6244dea679a913b
Tested-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Signed-off-by: Souvik Chakravarty <souvik.chakravarty@arm.com>