rdn2/run_model: create default pcie topology on IOMacro0.x16

In preparation to enable support for multiple IO Macros (on pciex16),
attach the minimal endpoint topology file to io_macro_0.pciex4_0 and
don't pass any topology file to io_macro_0.pciex16. By not passing any
topology file, FVP will create a default predefined PCIe topology with
multiple exercisers, switches, smmu test engines and ahci devices. Also,
update the ahci image path to use io_macro_0.pciex16 instead of
io_macro_0.pciex4.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I09ceee2461956698bf0ffd634fc19c4e40b30442