PCI: stm32-ep: Add PCIe Endpoint support for STM32MP25

PCI: stm32-ep: Add PCIe Endpoint support for STM32MP25

Add driver to configure the STM32MP25 SoC PCIe controller based on the
DesignWare PCIe core in endpoint mode. Controller support 2.5 and 5 GT/s
data rates and uses the common reference clock provided by the host.

The PCIe core_clk receives the pipe0_clk from the ComboPHY as input,
and the ComboPHY PLL must be locked for pipe0_clk to be ready.
Consequently, PCIe core registers cannot be accessed until the ComboPHY is
fully initialised and REFCLK is enabled and ready.

Signed-off-by: Christian Bruel <christian.bruel@foss.st.com>
[mani: reworded description]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
[bhelgaas: squash in https://patch.msgid.link/20250902122641.269725-1-christian.bruel@foss.st.com
to remove redundant link_status checks]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250820075411.1178729-7-christian.bruel@foss.st.com