Platform/Sgi: Add GetProductId API for SGI/RD Platforms

Add GetProductId API for SGI/RD Platform. The API returns a product id
in integer format based on the platform description data. The product id
is required for other drivers such as SMBIOS.

Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>
Reviewed-by: Thomas Abraham <thomas.abraham@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>