Blackfin arch: move hard coded pin_req to board file

Remove some sort of bloaty code, try to get these pin_req arrays built at compile-time

 - move this static things to the blackfin board file
 - add pin_req array to struct bfin5xx_spi_master
 - tested on BF537/BF548 with SPI flash

Signed-off-by: Bryan Wu <bryan.wu@analog.com>