Undo incorrect assumptions about Xen in commit 6ca0460f.

Xen invokes SeaBIOS via entry_post, not entry_elf.  As a result commit
6ca0460f broke Xen.  This change effectively undoes that commit.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Add README.CSM

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Add find_pmtimer() function

With CSM (and maybe coreboot) we'll want to find the pmtimer from the ACPI
tables, instead of knowing where it is and *putting* it into the ACPI
tables.

Extract the first part of the existing find_resume_vector() function into
a find_fadt() function, and use it from both find_resume_vector() and our
new find_pmtimer().

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Enable PMTIMER for CSM build

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>