gpio: davinci: Assign first bank regs for unbanked case

As per the re-design assign the first bank regs for unbanked
irq case. This was missed out in the original patch.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Fixes: b5cf3fd8 ("gpio: davinci: Redesign driver to accommodate ngpios in one gpio chip")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>