PCI/ASPM: Calculate and save the L1.2 timing parameters

PCI/ASPM: Calculate and save the L1.2 timing parameters

Calculate and save the timing parameters that need to be programmed if we
need to enable L1.2 substates later.

We use the same logic (and a constant value for 1 of the parameters) as
used by Intel's coreboot:

  https://www.coreboot.org/pipermail/coreboot-gerrit/2015-March/021134.html
  https://review.coreboot.org/#/c/8832/

Signed-off-by: Rajat Jain <rajatja@google.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>