adds pkg:github/Mbed-TLS/mbedtls to mbed_tls purls

[weichslgartner]

current github of mbed-tls is https://github.com/Mbed-TLS/mbedtls
This purl is also used by Zephyr (see https://github.com/zephyrproject-rtos/mbedtls/blob/zephyr/zephyr/module.yml) and causes partly intel/cve-bin-tool#4700

[scanossmining]

Hi @weichslgartner ,

Thank you for your input! However, I cannot merge this change as the purl must be lowercased, according to the purl specs: https://github.com/package-url/purl-spec/blob/master/PURL-TYPES.rst#github

[weichslgartner]

Hi @scanossmining,

thanks for the feedback, I changed it to lowercase (I fixed my commit instead of using the Github Review Tool).