fwu: add helper to get image GUID by type and bank index

fwu: add helper to get image GUID by type and bank index

Introduce fwu_mdata_get_image_guid() to retrieve a specific image GUID
from the FWU metadata based on the bank index and image type GUID.

This allows identifying the correct partition in multi-bank (A/B)
scenarios, ensuring the correct image is targeted depending on the
current bank.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>