PCI: imx6: Delay enabling reference clock for SS until it stabilizes

According to the IMX6 reference manuals, REF_SSP_EN (Reference clock enable
for SS function) must remain deasserted until the reference clock is
running at the appropriate frequency.

Delay enabling the reference clock for the SS function until it has
stabilized.  This prevents a high link failure rate (>5%) on certain IMX6
boards at various temperatures.

[bhelgaas: reword changelog slightly]
Tested-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Lucas Stach <l.stach@pengutronix.de>