Blackfin arch: fix bug - gpio_bank() macros messed up bank number caculating with positioning a gpio

The whole story:

Before BF51x merged, all the MAX_BLACKFIN_GPIOS are integral multiple of GPIO_BANKSIZE (= 16).
But BF51x provides MAX_BLACKFIN_GPIOS = 40 which includes 3 banks and the 3rd bank has only 8
GPIO pins.

Therefore, gpio_bank() macros is correct when you try to find a GPIO in which bank (GPIO_35 is
in bank 2). But on BF51x gpio_bank(MAX_BLACKFIN_GPIOS) only gives out 2 banks instead of 3
banks for some static array initialization.

This patch add a new macros gpio_bank_n() and GPIO_BANK_NUM to do bank number caculating and
remain the gpio_bank() macros for positioning a gpio in which bank.
Signed-off-by: Bryan Wu <cooloney@kernel.org>