drivers: interrupt_controller: imply the need for dynamic memory in ITS

The implementation of GIC v3 ITS uses k_aligned_malloc(), which will
only work if dynamic memory is available (system heap). Tell the user
that a dynamic memory pool is required.

The amount of memory will depend on registers probed during runtime.

Signed-off-by: Rodrigo Cataldo <rodrigo.cataldo@huawei.com>
Co-authored-by: Henri Xavier <datacomos@huawei.com>