mmc: sunxi: Enable eMMC HS-DDR (MMC_CAP_1_8V_DDR) support

Now that clock delay settings for 8 bit DDR are correct, and vqmmc
support is available, we can enable MMC_CAP_1_8V_DDR support. This
enables MMC HS-DDR at up to 52 MHz, even if signal voltage switching
is not available.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>