docs: use present tense when possible

Modify the sentences with past tense, progressive
tense or future tense to present tense for more
objective description if possible.

Signed-off-by: Joyce Kong <joyce.kong@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: I499e906e152c46ff356a521df3a08a7d3bd66733