nrf_bsim: nrf53_bsim_cpunet: Automatically start CPU

Automatically start the Network domain CPU during HW boot.
In the real HW this CPU is held on reset until something else
(Typically the application MCU) releases it.
But doing so facilitates running tests in this MCU as there is
no requirement for an image for the application core.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>