ARM: dts: OMAP2+: Use existing constants for GPIOs

Use standard GPIO constants to enhance the readability of DT GPIOs.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Benoit Cousson <benoit.cousson@linaro.org>