PCI: rockchip: Remove the pointer to L1 substate cap

Per the errata of TRM, the RC can't support L1 substate, so remove the L1
substate cap as well as operation for PCIE_RC_CONFIG_L1_SUBSTATE_CTRL2.

Tested-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>