Add support for the DesignWare MMC hardware in the HiSilicon hi6220. This
SoC is used in the HiKey board from 96boards.

Currently on the SD card is working on the HiKey, as such devices 0 and 2
will need to be disabled, for example by adding the following to
loader.conf:

hint.hisi_dwmmc.0.disabled=1
hint.hisi_dwmmc.2.disabled=1

Relnotes: yes (Hikey board booting)
Sponsored by: ABT Systems Ltd