input: it8xxx2_kbd: Respect kso_ignore_mask for all KSO pins

The kso_ignore_mask DTS property was not being fully applied in the
ITE IT8xxx2 keyboard driver:
 1. The driver performed direct writes to the KBS_KSOH2 register,
    which controls KSO[17:16]. This did not account for the
    kso_ignore_mask and could overwrite bits in the register that
    are used for other GPIO functions.
 2. The driver would attempt to initialize the GPIOs for KSO16 and
    KSO17 based solely on the col-size property. It ignored the
    kso_ignore_mask, which could lead to reconfiguring a pin that
    is intended for another purpose.

Signed-off-by: Wai-Hong Tam <waihong@google.com>