MLK-25349-1 dt-bindings: imx6q-pcie: add one regulator used to power up pcie phy

Both 1.8v and 3.3v power supplies can be used by i.MX8MQ PCIe PHY.
In default, the PCIE_VPH voltage is suggested to be 1.8v refer to data
sheet. When PCIE_VPH is supplied by 3.3v in the HW schematic design,
the VREG_BYPASS bits of GPR registers should be cleared from default
value 1b'1 to 1b'0. Thus, the internal 3v3 to 1v8 translator would be
turned on.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Reviewed-by: Jun Li <jun.li@nxp.com>
(cherry picked from commit c14681471c737280d93d1e5f83221576caf352ee)
(cherry picked from commit dc80c759ebb82f5ffc8e7ae263427b3b9d49d854)
Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>

Documentation/devicetree/bindings/pci/fsl,imx6q-pcie.txt

@@ -51,6 +51,9 @@ Optional properties:
   The regulator will be enabled when initializing the PCIe host and
   disabled either as part of the init process or when shutting down the
   host.
+- vph-supply: Should specify the regulator in charge of VPH one of the three
+  PCIe PHY powers. This regulator can be supplied by both 1.8v and 3.3v voltage
+  supplies. Might be used to distinguish different HW board designs.
 - ext_osc: use the external oscillator as ref clock( 1: external OSC is
   used, 0 internal PLL is used).
 - hard_wired: the PCIe port is hard wired to the EP device(0: one slot