Blackfin arch: Fix bug set correct baud for spi mmc and enable SPI after DMA.

Changes:

1. The baud for spi mmc defined in board file is not used by the old
spi driver. A slower value from spi framework is used instead. In latest
bug fixing, the correct baud is use which is too high for spi MMC card.

2. SPI is enabled only after DMA is started.

3. MMC detection IRQ is set to 55.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>