In preparation for moving the PPI enum out into a header, rename the
enum and its constituent values these so they are namespaced w.r.t. the
arch timer. This will aid consistency and avoid potential name clashes
when this move occurs.

No functional change.
Signed-off-by: NFu Wei <fu.wei@linaro.org>
Tested-by: NXiongfeng Wang <wangxiongfeng2@huawei.com>
[Mark: reword commit message]
Signed-off-by: NMark Rutland <mark.rutland@arm.com>