Make TFTF RFC 4122 compliant

This is a TFTF backport of a change that makes TF RFC 4122-compliant
by converting the stored format of UUIDs from machine order (little
endian) to network order (big endian).

This patch changes the data structure used to store the values in the
same way as in the related change in TF:
033648652f2d66abe2454a75ded891a47cb13446.

Signed-off-by: Oliver Swede <oli.swede@arm.com>
Change-Id: I052e570b80de61f87a049a08e347a2e5da7f841b