riscv: support Allwinner D1 MMC controller

[julien.cassette_gmail.com]

Summary

This was a work in progress but I cannot get it to work, so just
submitting it if someone has an idea or wants to debug it...

Modify the aw_mmc driver for the D1:

• BUFF_SIZE must be a multiple of 4 bytes
• BUFF_ADDR is a word address
• NEXT_DESP_ADDR is a word address
• DES_BASE_ADDR is a word address

Signed-off-by: Julien Cassette <[e-mail]>