refactor(realm): define PCIe helpers

This patch refactors the existing PCIe and DOE
helpers to define generic helpers to make them
reusable across more tests.

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: I56a9f5c59715c7916f3f737ed6d3af94b0e3679f