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>