PCI: dwc: Implement generic suspend/resume functionality

Introduce an helper function (dw_pcie_get_ltssm()) to retrieve
SMLH_LTSS_STATE.

Add common dw_pcie_suspend(resume)_noirq() API to implement the DWC
controller generic suspend/resume functionality.

Add a controller specific callback to send the PME_Turn_Off message
(ie .pme_turn_off) for controller platform specific PME handling.

Link: https://lore.kernel.org/r/20230821184815.2167131-3-Frank.Li@nxp.com


Signed-off-by: Frank Li <Frank.Li@nxp.com>
[lpieralisi@kernel.org: commit log]
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Acked-by: Manivannan Sadhasivam <mani@kernel.org>