lenovo/x60: CST table: use MWAIT requests instead of P_LVLx I/O reads

lenovo/x60: CST table: use MWAIT requests instead of P_LVLx I/O reads

Requesting low power acpi cpu c-states has two software interfaces:
Using P_LVLx I/O reads or using equivalent MWAIT requests.
This change makes it more consistent with newer targets that use MWAIT
requests.

There also exists extended intel acpi c-states which can be enabled
in two ways:
- using a substate hint to the mwait request (defined in bios);
- setting a model specific register (msr)

Currently this is done by setting the right msr bits but with this
change one can experiment by adding substate hints.

Change-Id: I9eeb5b008e2ddc2193725667f2c13582a4877e3c
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/14801
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>