gpt: fix partion size limit

Currently, in gpt command, partion size is converted from string
to unsigned long type using 'ustrtol' function. That type limits
the partition size to 4GB.

This patch changes the conversion function to 'ustrtoll' to return
unsigned long long type.

Signed-off-by: Piotr Wilczek <p.wilczek@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>