cmd/gpt: Support gpt command for all devices

The gpt command was only implemented for mmc devices. There is no reason
why this command should not be generalized and be applied all other
storage device classes.
This change both simplifies the implementation and eliminates a
build failure for systems that don't support mmcs.

Signed-off-by: Egbert Eich <eich@suse.com>
Tested-by: Piotr Wilczek <p.wilczek@samsung.com>
[trini: Change coding style slightly]
Signed-off-by: Tom Rini <trini@ti.com>