Per errata i2104 documented in AM65x device errata document (TI document
number SPRZ452E, revised June 2019), Gen3 operation is not supported for
both PCIe Root Complex and Endpoint modes of operation.

See: https://www.ti.com/lit/er/sprz452e/sprz452e.pdf

Restrict speed to Gen2 to address the errata.
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20200802165356.10285-1-nsekhar@ti.com