PCI: Add #defines for Enter Compliance, Transmit Margin

Add definitions for the Enter Compliance and Transmit Margin fields of the
PCIe Link Control 2 register.

Link: https://lore.kernel.org/r/20191112173503.176611-2-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>