clk: mediatek: Make reset_control_ops const

The mtk_reset_ops structure is never modified. Make it const.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>