ec/lenovo/h8: Add function to query sense state

ec/lenovo/h8: Add function to query sense state

* Add function to wait for sense registers to become valid.
* Add function to retrieve Fn-Key state.

Tested on Lenovo T500:
* It takes about 700msec for the registers to become valid.

Tested on Lenovo T520:
* It takes less than 150msec for the registers to become valid.

Change-Id: Ie27e2881a256c4efb3def11f05070c446db6e5fc
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/29204
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>