bluetooth: controller: define generic macros for LLL, ULL LOW IRQs

This commit defines generic macros for the software interrupt
(SWI) IRQ lines that are used for LLL and ULL LOW interrupts.
This is done for both the cases of the legacy and new (split)
controller architectures. In addition, it abstracts some of
the functionality around software-IRQ signals, to generic
functions, which have platform-specific implementations.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>