mmc: sunxi: Support MMC DDR52 transfer mode with new timing mode

The MMC controller can support DDR52 transfers under the new timing
mode. According to the BSP kernel, the module clock has to be double
the card clock, regardless of the bus width. The default timings in
the hardware can be used.

This also reworks the code setting the internal divider, getting rid
of a extra conditional.

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>