device/pciexp_device: Set values numerically instead of as bitmask

As noted on linux-pci, we have a weird way to handling "value" and
"scale" fields that are supposed to contain numerical values: we encode
them as a bitfield.
Instead define the two fields (offset and mask) and use numbers.

Another issue, not fixed in this CL, is that we write hard-coded values
while these fields really need to contain the max() of acceptable delays
of the downstream devices. That way the controller can decide whether or
not to enter a deeper power management state. It's noted as a TODO.

Change-Id: I895b9fe2ee438d3958c2d787e70a84d73eaa49d2
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Found-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-on: https://review.coreboot.org/22740
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/include/device/pciexp.h

@@ -9,6 +9,11 @@ enum aspm_type {
 	PCIE_ASPM_BOTH = 3,
 };
 
+#define ASPM_LTR_L12_THRESHOLD_VALUE_OFFSET 16
+#define ASPM_LTR_L12_THRESHOLD_VALUE_MASK (0x3ff << ASPM_LTR_L12_THRESHOLD_VALUE_OFFSET)
+#define ASPM_LTR_L12_THRESHOLD_SCALE_OFFSET 29
+#define ASPM_LTR_L12_THRESHOLD_SCALE_MASK (0x7 << ASPM_LTR_L12_THRESHOLD_SCALE_OFFSET)
+
 void pciexp_scan_bus(struct bus *bus, unsigned int min_devfn,
 			     unsigned int max_devfn);