Docs: Update path 'services' to 'partitions'

Due to the folder name is updated from 'secure_fw/services' to
'secure_fw/partitions', update related documents with updated name.

Change-Id: I5b85d3c2c4f2a4e1f7fd422910e968e970d70527
Signed-off-by: Ken Liu <ken.liu@arm.com>