AmpereAltraPkg: Correct PCIe Devmap

This corrects the high devmap of Root Complex (RC)
that should be fetched from a high bifurcation.

Signed-off-by: Nhi Pham <nhi@os.amperecomputing.com>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>