docs(auth): add auth_decrypt in CM chapter

The call to REGISTER_CRYPTO_LIB requires auth_decrypt function to be
provided. Add its prototype and update REGISTER_CRYPTO_LIB call.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: Id1f2a54867ffe5dec36e0bf22490d01858891585
1 file changed