The comment suggests checking version numbers and
a checksum but there doesn't seem to be any usable
data for either of these.

For example, fip_toc_header_t doesn't contain any
version information and neither does fip_toc_entry_t.

As the function name "is_valid_header" suggests, this
function is not concerned with checksumming any of
the table of contents entries.

Change-Id: I8673ae5dd37793771760169f26b2f55c15fbf587
Signed-off-by: Paul Beesley <paul.beesley@arm.com>