mbedtls: support configuration of PSK maximum length via Kconfig

So far maximum PSK length was configured by mbedTLS as 32 bytes.
Introduce Kconfig option that will configure it instead of relying on
default value from mbedTLS library, so that user can easily adjust that
value to application needs.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>