Bluetooth: controller: nRF53: Use SWI instead of EGU

nRF5340 does have SWI peripheral, hence use it instead of
the single available EGU peripheral. Use of SWI will allow
controller's LLL, ULL_HIGH and ULL_LOW execution context to
be independently configured to different interrupt priority
levels.

When ULL_HIGH priority equals ULL_LOW priority, only SWI2
is used by controller. Otherwise, SWI3 is used for ULL_LOW.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>