Blackfin arch: Convert Blackfin GPIO driver to use common gpiolib/gpiochip infrastructure

 - This patch adds support for ARCH_WANT_OPTIONAL_GPIOLIB.
 - It may be changed in future to ARCH_REQUIRE_GPIOLIB.
 - Change GPIO_BANK_NUM use DIV_ROUND_UP( , ) macro

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>