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 <alberto.escolar.piedras@nordicsemi.no>