refactor(lib/attestation): make private-use functions private

Some public functions in the attestation module are not used by
any module other the attestation one, so there is no need for
them to be public. This patch moves those functions to the proper
private header file so the functions become private for the
attestation module.

Change-Id: I2d6040f0c66a3be6018a6cbe0743dca9da5cc551
Signed-off-by: Juan Pablo Conde <juanpablo.conde@arm.com>