tf_config(mbedtls): test out MBEDTLS_CONFIG_FILE usage

commit:
("8841f11a refactor: mbedtls: Remove redundant MBEDTLS_CONFIG_FILE")

Removed usage of MBEDTLS_CONFIG_FILE to make way for mbedtls3 testing,
But later it was pointed out during discussions MBEDTLS_CONFIG_FILE
is a valid build option and currently might be untested in our CI.

So add a simple usage of MBEDTLS_CONFIG_FILE with mbedtls-3 config
file.

Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
Change-Id: Ib0aa07d2307094e21db30764ae8912c9ef3d344e
(cherry picked from commit 31ed053e761d8cbb580b9ee51b11b541e77d1827)
1 file changed