clk: sunxi: sun9i-mmc: Implement reset callback for reset controls

Our MMC host driver now issues a reset, instead of just deasserting
the reset control, since commit c34eda69 ("mmc: sunxi: Reset the
device at probe time"). The sun9i-mmc clock driver does not support
this, and will fail, which results in MMC not probing.

This patch implements the reset callback by asserting the reset control,
then deasserting it after a small delay.

Fixes: 7a6fca87

 ("clk: sunxi: Add driver for A80 MMC config clocks/resets")
Cc: <stable@vger.kernel.org> # 4.14.x
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20171218035751.20661-1-wens@csie.org