scripts: gen_isr_tables: Code cleanup

This commit cleans up the gen_isr_tables code for better clarity
and easier modification.
Changes include:
- Separate functions to load intList section.
- Replace spurious handlers in internal data with None.
  Now it is the output generator work to choose right function.
- All the work to generate vt and swt separated into its own functions.
- Remove the need for internal shared array - all the information
  is here in swt array.
- The update_masks function - more functionality moved here.
- Simplify bit_mask function.

Signed-off-by: Radosław Koppel <radoslaw.koppel@nordicsemi.no>