SPM: Rename IS_PARTITION_*** to IS_***

Remove redundant namespace 'PARTITION' to make it shorter.
And updated the flags bit assignment explanation comments for easier understanding.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: I218e08b945309017f07e2e595a3c684e00be4bdf
17 files changed