mmc: sunxi: Disable eMMC HS-DDR (MMC_CAP_1_8V_DDR) for Allwinner A80

eMMC HS-DDR no longer works on the A80, despite it working when support
for this developed.

Disable it for now.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>