SPM: Rename 'spm/common' into 'spm/ffm'

The name 'common' confused a bit, as its real purpose is for general
FF-M API implementation. Rename this folder as the first step for
re-structure patches.

There are some 'common' folder legacies still in the renamed 'ffm'
folder. They will be updated in the following patches.

Change-Id: Ic72fcf9ecf47f2185a82761edfdcbcd3c4569421
Signed-off-by: Ken Liu <Ken.Liu@arm.com>
18 files changed