refactor(auth): replace plat_convert_pk

refactor(auth): replace plat_convert_pk

Following discussions in the reviews of the patch that introduced
plat_convert_pk() function [1], it was decided to deprecate it to
avoid weak function declaration.
A new optional function pointer convert_pk is added to crypto_lib_desc_t.
A new function crypto_mod_convert_pk() will either call
crypto_lib_desc.convert_pk() if it is defined, or do the same
as what was done by the weak function otherwise.

[1] https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/17174

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I9358867f8bfd5e96b5ee238c066877da368e43c6