refactor(crypto): avoid using struct mbedtls_pk_rsassa_pss_options

refactor(crypto): avoid using struct mbedtls_pk_rsassa_pss_options

In preparation for supporting mbedtls 3.3, usage of
mbedtls_pk_rsassa_pss_options[1] is made private and is broken on 3.3

However looking closely into the usage in 'verify_signature' function
is no hard reason behind usage of this struct and they could be easily
replaced with independent variables.

This Minor refactor to avoid using the struct mbedtls_pk_rsassa_pss_options
and use independent variable will provide compatibility with both 2.x
and 3.x

[1]: https://github.com/Mbed-TLS/mbedtls/issues/7040

Change-Id: If0107d860d11d13cba7fd5d7941e7142e70c7b11
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>