Platform:corstone1000: Introduce GPT parser

Adding GPT parser
Side changes required:
Includes the implementation of the `plat_get_image_source` function
in the platform.c file.

The GPT parser requires the function. Given the image id, it should
return handle to the IO device contains the image and image
specification that allows IO storage access to the image.

Signed-off-by: Mohamed Omar Asaker <mohamed.omarasaker@arm.com>
Change-Id: I3ff32c83fbc589c993f3f069985c0bae9efdad0a