PCI: host-common: Convert to library for host controller drivers

PCI: host-common: Convert to library for host controller drivers

This common library will be used as a placeholder for helper functions
shared by the host controller drivers. This avoids placing the host
controller drivers specific helpers in drivers/pci/*.c, to avoid enlarging
the kernel image on platforms that do not use host controller drivers at
all (like x86/ACPI platforms).

Suggested-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250508-pcie-reset-slot-v4-3-7050093e2b50@linaro.org