DPE: Docs: Replace layer terminology with certificate

DPE spec does not have a notion of layers. Replace layer terminology
with certificate (or cert) in the code base.

Signed-off-by: Maulik Patel <maulik.patel@arm.com>
Change-Id: Ia5723b422f8a0499a051556b9bfdd88de5e1af41
3 files changed