gpio: omap: Give unique labels to each GPIO bank/chip

As we need to add GPIO lookup tables to the OMAP platforms, we
need to reference each GPIO chip with a unique label. Use the GPIO
base to name each chip, "gpio-0-31", "gpio-32-63" etc.

Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: Santosh Shilimkar <ssantosh@kernel.org>
Cc: Kevin Hilman <khilman@kernel.org>
Cc: linux-omap@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>