MFC r305689:

a10_mmc: Remove completly the PIO code now all access is done by DMA.
Rename registers as in the manual.
Do a hard reset of the controller before a soft one.
Since DMA is always used remove dependancy on allwinner_soc_family, it was used
to differentiate SoC as the fdt compatible string were the same.

Tested on A10, A20, H3 and A64.

Reviewed by: jmcneill
Differential Revision: https://reviews.freebsd.org/D6868