mmc: mmc_spi: Re-write driver using DM framework

This patch rewrites MMC SPI driver using U-Boot DM
framework and get it's working on SiFive Unleashed
board.

Signed-off-by: Bhargav Shah <bhargavshah1988@gmail.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>