[ACPI] Add "ec_polling" boot option

EC burst mode benefits many machines, some of
them significantly.  However, our current
implementation fails on some machines such
as Rafael's Asus L5D.

This patch restores the alternative EC polling code,
which can be enabled at boot time via "ec_polling"

http://bugzilla.kernel.org/show_bug.cgi?id=4665

Signed-off-by: Luming Yu <luming.yu@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>