PCI: dra7xx: Add support to force RC to work in GEN1 mode

PCIe in AM57x/DRA7x devices is by default configured to work in GEN2 mode.
However there may be situations when working in GEN1 mode is desired.  One
example is limitation i925 (PCIe GEN2 mode not supported at junction
temperatures < 0C).

Add support to force Root Complex to work in GEN1 mode if so desired, but
don't force GEN1 mode on any board just yet.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>