Bluetooth: controller: Use direct ISRs

The new hard real-time (direct) interrupt support in the kernel allows
ISRs to be run directly with no significant latency overhead.

Use this new mechanism to improve the latency of the 3 critical
interrupts in the controller: radio, RTC0 and SWI4.

Jira: ZEP-1038

Change-Id: Ief9dacbea4c4c2e8a1c77893a0d6175a91819ffb
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>