mmc: sunxi: allow 3.3V DDR when DDR is available

Some Allwinner boards feature an on-board eMMC with fixed 3.3V voltage
(e.g. Banana Pi M2+), and in this case both the eMMC and the SoC are
capable of doing 3.3V DDR transmission.

Add capability of 3.3V DDR when DDR is available (extra clock or new
timing).

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>