Add suspend and resume support. Only the Root Complex mode is supported.

During the suspend stage PERST# is asserted, then deasserted during the
resume stage.

Link: https://lore.kernel.org/linux-pci/20240102-j7200-pcie-s2r-v7-7-a2f9156da6c3@bootlin.comSigned-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Signed-off-by: Thomas Richard <thomas.richard@bootlin.com>
[kwilczynski: commit log, update references to the PCI SIG specification]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Siddharth Vadapalli <s-vadapalli@ti.com>