blackfin: add bf6xx spi driver

Spi driver for bf60x is different from old one, so implement a new
driver for it.

Signed-off-by: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Sonic Zhang <sonic.adi@gmail.com>