input: it8xxx2_kbd: add a kso-ignore-mask property

The it8xxx2_kbd KSO pins can be used as both keyboard scan and GPIO. By
default the keyboard scanning driver controls the output level of all
the KSO signals from 0 to (col-size - 1), meaning that any line in
between used as GPIO is going to have its output value overridden.

Add a kso-ignore-mask property to the keyboard scan driver to allow
specifiying extra pins that should not be controlled by the driver.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>