smcf unit_test: Create module unit tests

This patch adds unit tests for both mgi.c and mod_smcf.c.
Each test is in a separate directory. The tests for mod_smcf.c
requires mocking the mgi.c functions. The required mocks are
generated using the mock generator script gm.rb in unit_test
directory and saved under smcf/test/mocks directory.

Signed-off-by: Tarek El-Sherbiny <tarek.el-sherbiny@arm.com>
Co-authored-by: Abhishek Rawat <abhishek.rawat@arm.com>
Change-Id: I33250e35b9c58fb26166084fbbf1ef0df4c4f4c2