modules: mbedTLS: Add separate Kconfig entry for MBEDTLS_HAVE_TIME_DATE

Currently the MBEDTLS_HAVE_TIME_DATE mbedTLS option is enabled based on
the CONFIG_POSIX_API option. This doesn't seem right, since the enabling
the POSIX API does not guarantee that there is a valid time source in
the system. This was the case for the qemu_x86 platform, where enabling
POSIX_API caused TLS handshake failures due to certificate validation
errors caused by no valid time avaialble in the system.

Fix this by adding a specific KConfig entry for date/time configuration
in mbedTLS. Applications that need to enforce date verification in
mbedTLS should enable it explicitly instead of relying on the
non-obvious implicit configuration.

Fixes #35401

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>