[Blackfin] arch: GPIO API cleanup and anomaly update

 - Add anomaly workaround for bfin_gpio_reset_spi0_ssel1
 - Fix style
 - Update copyright
 - Remove BUG_ON checks for functions intended to be used only by arch
   support. GPIO users should only access using the generic GPIO API
 - Make all GPIO identifier unsigned int

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>