pci: mx6: fix occasional link failures

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.

Without this patch we find a high link failure rate (>5%) on certain
IMX6 boards at various temperatures.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>