PCI: starfive: Use regulator APIs to control the 3v3 power supply of PCIe slots

PCI: starfive: Use regulator APIs to control the 3v3 power supply of PCIe slots

The driver has been using the "enable-gpios" property to control the 3v3
power supply of PCIe slots. But it is not documented in the dt-bindings and
also using GPIO APIs is not a standard way to control PCIe slot power, so
use the documented "vpcie3v3-supply" property and regulator APIs to control
the slot supply.

This change will break the DTs which used "enable-gpio" or "enable-gpios"
property under the controller node. Since these properties were not defined
in the bindings, it is safe to switch to "vpcie3v3-supply". Any out-of-tree
DTS impacted by this change should migrate to "vpcie3v3-supply" instead.

Signed-off-by: Hal Feng <hal.feng@starfivetech.com>
[mani: reworded description]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Acked-by: Kevin Xie <kevin.xie@starfivetech.com>
Link: https://patch.msgid.link/20251218102149.28062-1-hal.feng@starfivetech.com