modules: Kconfig.tls-generic: Add CONFIG_MBEDTLS_ECDSA_DETERMINISTIC

From mbedTLS's description:

Enable deterministic ECDSA (RFC 6979).
Standard ECDSA is "fragile" in the sense that lack of entropy when
signing may result in a compromise of the long-term signing key. This
is avoided by the deterministic variant.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>