uboot-imx

gpio: Use more command-specific enums values

At present this file uses GPIO_OUTPUT and GPIO_INPUT as its sub-command
values. These are pretty generic names. Add a 'C' suffix to avoid possible
conflicts.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>