clk: sunxi-ng: a83t: Support new timing mode for mmc2 clock

The MMC2 clock supports a new timing mode. When the new mode is active,
the output clock rate is halved.

This patch sets the feature flag for the new timing mode, and adds
a pre-divider based on the mode bit.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>