module/ut: Add module for unit tests using unity on target

This commits adds a unit test module which can be used to
execute Unity/CMock based tests on target hardware/model.

This module basically links to unity/cmock framework
and calls specific tests from it's init function.

As an example few SCMI tests are added to demonstrate
the use of this module.

Please note that, SCP must run bare minimum SCP firmware(bl2)
with at least with one UART. Subsequent commit will demonstrate
a bare minimum bl2 firmware on the Juno board that include
module/ut

Change-Id: If20731a0e82240c47f6df4a41a1e6253c0e79a07
Signed-off-by: Girish Pathak <girish.pathak@arm.com>