Blackfin arch: a few things still use bfin_read_PORT_FER()

 - Update gpio_request to allow multiple request with the same signature (label)
 - Use generic GPIO API where applicable
 - Update generic board support form stamp board

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