PCI: rockchip: Advertise 128-byte Read Completion Boundary support

Rockchip Root Ports support either 64 or 128 byte Read Completion Boundary
(RCB).  Set the RCB bit in the Link Control register to indicate this.

A 128 byte RCB significantly improves performance of NVMe with libaio.

[bhelgaas: changelog]
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Brian Norris <briannorris@chromium.org>
Cc: Jeffy Chen <jeffy.chen@rock-chips.com>