rdinfra/pcie_hierarchies: add a minimal pcie hierarchy file

Add a minimal pcie hierarchy file with exerciser as endpoint for the
use of latest RD platforms.

Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
Change-Id: I72699e84f2fc751f266bb537dffec57bc232cf75

rdinfra/pcie_hierarchies/exerciser_minimal_ep.json

+{
+   "exerciser/exerciser": {
+   }
+}