arm: mvebu: theadorable: Enhance "pcie" test cmd to check link width/speed

This patch changes the board specific "pcie" U-Boot command to not only
check for PCIe device existance but also for the correct link speed
and width that has been established. This cmd can be used by U-Boot
scripts for automated testing, if the PCIe setup is correct. Meaning,
that all PCIe devices are correctly detected and the link speed and
width is corrent.

Signed-off-by: Stefan Roese <sr@denx.de>