input: it8xxx2_kbd: mask out KSOL and KSOH1 register access

KSOL and KSOH1 registers are used not only by the kbd driver, but
potentially by other GPIOs attached to the keyboard scanning pins. Mask
write access to those registers to ensure drivers don't step over each
other.

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