PCI: dw-rockchip: Configure L1SS support

PCI: dw-rockchip: Configure L1SS support

L1 PM Substates for RC mode require support in the dw-rockchip driver
including proper handling of the CLKREQ# sideband signal. It is mostly
handled by hardware, but software still needs to set the clkreq fields
in the PCIE_CLIENT_POWER_CON register to match the hardware implementation.

For more details, see section '18.6.6.4 L1 Substate' in the RK3568 TRM 1.1
Part 2, or section '11.6.6.4 L1 Substate' in the RK3588 TRM 1.0 Part2.

[bhelgaas: set pci->l1ss_support so DWC core preserves L1SS Capability bits;
drop corresponding code here, include updates from
https://lore.kernel.org/r/aRRG8wv13HxOCqgA@ryzen]

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/1761187883-150120-1-git-send-email-shawn.lin@rock-chips.com
Link: https://patch.msgid.link/20251118214312.2598220-4-helgaas@kernel.org