rdn2/run_model: create default pcie topology on pcie_group_0.x16

Before 11.15, if any hierarchy file is not passed to pcie_group_* port,
default pcie topology will be created. Starting from 11.15, if no file
is passed, FVP will not create any pcie hierarchy. To create FVP's
default pcie hierarchy corresponding port has to be set to <default>. On
both RD-N2, IO macro 0's x16 is used to create default pcie hierarchy.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: Ibf4343ccdeeeab0808b1745a98b0bf557551449d