pci: pcie_imx: Fix hang on mx6qp

PCI driver currently hangs on mx6qp.

Toggle the reset bit with the appropriate timings to fix the issue.

Based on the FSL kernel driver implementation.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>