ACPICA: clear fields reserved before FADT r3

Linux-2.6.21 stopped booting on a P4/HT because Linux
wrote the FADT.CST_CNT value to the SMI_CMD.
Apparently this stumbled over some SMM instability,
such as confusing SMM when invoking it from cpu1.

Linux did this because even though the r2 FADT reserves
the CST_CNT field, this BIOS set that field and Linux
used it.

Turns out that up through 2.6.20 we explicitly cleared
cst_control for r2 FADTs.  So here we go back to doing that,
plus also clear some additional fields that are reserved
until FADT r3.

http://bugzilla.kernel.org/show_bug.cgi?id=8346

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>