clk: sunxi-ng: Implement reset control status readback

Until now we were not providing a way to read back the status of our
reset controls. Consumers had no real way to be certain whether a
peripheral was held in reset or not.

Implement the status callback to complete the API support.

Fixes: 1d80c142 ("clk: sunxi-ng: Add common infrastructure")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>