device: Add flag to disable PCIe ASPM

For broken devices that spuriously advertise ASPM, make it possible to
decide ASPM activation in the device driver.

Change-Id: I491aa32a3ec954be87a474478609f0f3971d0fdf
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/25617
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/device/pciexp_device.c

@@ -377,6 +377,9 @@ static void pciexp_enable_aspm(device_t root, unsigned root_cap,
 	u16 lnkctl;
 	u32 devcap;
 
+	if (endp->disable_pcie_aspm)
+		return;
+
 	/* Get endpoint device capabilities for acceptable limits */
 	devcap = pci_read_config32(endp, endp_cap + PCI_EXP_DEVCAP);