PCI: rcar: Try increasing PCIe link speed to 5 GT/s at boot

The PCIe link speed is initially set to 2.5 GT/s.  Try to increase the link
speed to 5 GT/s.

Based on original patch by Grigory Kletsko
<grigory.kletsko@cogentembedded.com>.

[bhelgaas: remove "Trying speed up" message, remove unused SPCHG]
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>