PCI: intel-gw: Enable clock before PHY init

PCI: intel-gw: Enable clock before PHY init

To ensure that the boot sequence is correct, the DWC PCIe core clock must
be switched on before PHY init call [1]. This changes are based on patched
kernel sources of the MaxLinear SDK.

The reason why the MaxLinear SDK is used as a reference here is, that this
PCIe DWC IP is used in the URX851 and URX850 SoC. This SoC was originally
developed by Intel when they acquired Lantiq’s home networking division in
2015 [2]. In 2020 the home network division was sold to MaxLinear [3].
Since then, this SoC belongs to MaxLinear. They use their own SDK, which
runs on kernel version '5.15.x'.

[1] https://github.com/maxlinear/linux/blob/updk_9.1.90/drivers/pci/controller/dwc/pcie-intel-gw.c#L544
[2] https://www.intc.com/news-events/press-releases/detail/364/intel-to-acquire-lantiq-advancing-the-connected-home
[3] https://investors.maxlinear.com/press-releases/detail/395/maxlinear-to-acquire-intels-home-gateway-platform

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260417-pcie-intel-gw-v5-4-0a2b933fe04f@dev.tdt.de