modules: tfm: add configuration for disabling the RNG module

A new configuration is available in TF-M, for disabling RNG
independent from key derivation. Introduce a corresponding
Kconfig option to control the RNG API enabling/disabling from
Zephyr.

Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>