ARM: dts: imx6qdl-sabresd: Use the 'vpcie-supply' property

Since commit c26ebe98 ("PCI: imx6: Add regulator support"), it is
possible to pass the 'vpcie-supply' property to describe the PCIE supply.

This way we can remove the 'regulator-always-on' property from the
regulator and have a better device tree description.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>