extcon: palmas: Make of_device_id array const

Array of struct of_device_id may be be const as expected by
of_match_table field.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: Graeme Gregory <gg@slimlogic.co.uk>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>