[Blackfin] arch: Encourage users to use the spidev character driver: Provide platform support

 - Enable kernel generic spidev driver for blackfin SPI ADC
 - spi_adc driver, document and test sample not synced

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