MLK-10009-7 PCI: imx6: Wait the clocks to stabilize after ref_en

For boards without a reset gpio we skip the delay between enabling
the pcie_ref_clk and touching the RC registers for configuration.
System would be hangs when the clocks are not yet settled in the DW
PCIe core. So we need to make sure that there is always an
appropriate delay between those two actions.

Signed-off-by: Richard Zhu <richard.zhu@freescale.com>