pinctrl: nomadik: use BIT() with offsets consequently

This driver is confusing in referencing/dereferencing the global
GPIO number scope in some places and using local offsets in other
places. Remove some of the confusion by removing local "bit" and
"bitmask" definitions and just use BIT(offset) directly. Also
unexport a function only used in this file.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>