trivial: Fix compiler warnings

- Declare unused variables
- Move variable initialization to prevent uninitialized use
- Correctly store bitwise operation result
- Fixup memset calls to use correct size
- Add missing return statement to functions
- Conditionally define functions that are used conditionally

libsel4platsupport: Add mini IRQ interface

The new mini IRQ interface covers the common use case where there's only
a few interrupts to manage and these can fit into a single
notification's badge space. This also avoids the unneeded complexity
that the standard IRQ interface offers.

libsel4platsupport: Refactor the IRQ sources

This commit refactors the IRQ sources and pulls out common code into
separate functions. This is in preparation of a more simpler/mini IRQ
interface.

trivial: style changed files

libsel4platsupport: Improve IRQ interface config

To make it easier to pass default options to the IRQ interface, the max
IRQ and notification IDs options are combined into a single struct. Also
define a simple default option to streamline the process of creating the
IRQ interface.

libsel4platsupport: Update IRQ callback type

Following the change in the IRQ interface API in libplatsupport, this
commit updates the IRQ interface sources to reflect the type name
change.

libsel4platsupport: Add new IRQ implementation

The commit adds the IRQ interface implementation. The implementation
also includes the functions that are required by the base IRQ interface
in libplatsupport.