PCI: imx6: Increase link startup timeout

A longer link startup timeout is required when certain PCI switches are
attached to the root complex.  This was tested with a Pericom switch
and a PLX switch.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>