PCI: keystone: Fix ks_pcie_v3_65_add_bus() for AM654x SoC

In the process of converting .scan_bus() callbacks to .add_bus(), the
ks_pcie_v3_65_scan_bus() function was changed to ks_pcie_v3_65_add_bus().
However, ks_pcie_v3_65_scan_bus() belonged to:
"struct dw_pcie_host_ops ks_pcie_host_ops"
which is specific to DW PCIe IP-core version 3.65a. On the other hand,
ks_pcie_v3_65_add_bus() has been added to:
"struct pci_ops ks_pcie_ops"
which is shared with DW PCIe IP-core version 4.90a as well.

AM654x SoC uses DW PCIe IP-core version 4.90a and ks_pcie_v3_65_add_bus()
is not applicable to it and shouldn't be invoked. Thus, make use of the
"is_am6" flag and change ks_pcie_v3_65_add_bus() to no-op for AM654x SoC.

Fixes: 6ab15b5e

 ("PCI: dwc: keystone: Convert .scan_bus() callback to use add_bus")
Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>