nrf5340bsim: Add support for NRF53_SYNC_RTC & ENABLE_CPUNET

Add support for these two options in the simulated board.

Before these options were otherwise defined in the real
BOARD/SOC kconfig files, which meant samples/tests which
used them would not be able to build due to a kconfig error.

With this change they can both be selected, and the right
functionality is built in.

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