Input: da9063_onkey - avoid using OF-specific APIs

There is nothing OF-specific in the driver, so switch from OF properties
helpers to generic device helpers.

Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/ZYOsUfKceOFXuCt5@google.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>