PCI: imx6: Add initial imx6sx support

Add initial PCIe support for the imx6 SoC derivate imx6sx.  PCI MSI support
is untested as the necessary suspend/resume quirk is not included in this
patch.

This patch is heavily based on patches by Richard Zhu.

[bhelgaas: factor out refclk enable, fix adjacent typos in imx6q-pcie.txt]
Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Acked-by: Richard Zhu <Richard.Zhu@freescale.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>