ENGR00275213-1 arm: pcie: enable pcie on imx6 platforms

ENGR00275213-1 arm: pcie: enable pcie on imx6 platforms

Add PCIe related clocks definitions and select pci on imx6
platforms

toggle bit18 of grp1 fix pcie pm issue:
Set bit18 of gpr1 before enter into supend, and clean it
after resume, can fix the following errata.
Errata ERR005723_PCIe PCIe does not support L2 Power Down.

Signed-off-by: Richard Zhu <r65037@freescale.com>