mmc: sunxi: add support for A64 mmc controller

A64 SoC features a MMC controller which need only the mod clock, and can
calibrate delay by itself. This patch adds support for the new MMC
controller IP core.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>