feat(optee): check paged_image_info

For OP-TEE without pager, the paged image may not be present in OP-TEE
header. We could then pass NULL for paged_image_info to the function
parse_optee_header(). It avoids creating a useless struct for that
non existing image. But we should then avoid assigning header_ep args
that depend on paged_image_info.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I4fdb45a91ac1ba6f912d6130813f5215c7e28c8b