boards: nrf54l15bsim doc: Clarify around entropy and crypto

Let's explicitly mention we are enabling the native_posix entropy
driver for this board (the real nrf54l15dk does not have an entropy
driver in Zephyr yet).
And correct a bit the wording around mbedtls as for the real target we
don't have mbedtls with HW acceleration.

Signed-off-by: Alberto Escolar Piedras <[email protected]>

Author: aescolar

boards/native/nrf_bsim/doc/nrf54l15bsim.rst

@@ -89,5 +89,6 @@ ARM's TrustZone is not modeled in this board. This means that:
   can be labeled as restricted for secure or non secure access.
 * TF-M cannot be used.
 
-Note that the CRACEN peripheral is not modeled. The mbedTLS library can still be used
-but with a SW crypto backend.
+Note that the CRACEN peripheral is not modeled.
+As crypto library, Mbed TLS can be used with its SW crypto backend.
+As entropy driver, the :dtcompatible:`zephyr,native-posix-rng` is enabled by default.