MLK-10009-3 ARM: imx6: Update dts and binding for imx6sx pcie

- imx6sx pcie phy has its own power regulator. Add the
pcie phy power suppy into im6sx pcie dts and binding.
- in order to align with imx6qdl's pcie dts, re-format
imx6sx pcie dts.
- in order to align with imx6qdl pcie dts format and
keep clean of imx6 pcie driver, keep the pcie phy clock
in imx6sx pcie dts, although it's the parent clk of the
pcie bus clock now, and would be enabled automatically
when pcie bus clock is enabled. secondly, it's
possible that the external osc maybe used as source
of the pcie_bus clk in board design in future.
- disp_axi clock is required by pcie inbound axi port.
Add one more clock named pcie_inbound_axi for imx6sx pcie.

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