arch: arm: aarch32: Refactor interrupt interface

The current AArch32 (Cortex-R and to-be-added Cortex-A) interrupt
system relies on the multi-level interrupt mechanism and the
`irq_nextlevel` public interface to invoke the Generic Interrupt
Controller (GIC) driver functions.

Since the GIC driver has been refactored to provide a direct interface,
in order to resolve various implementation issues described in the GIC
driver refactoring commit, the architecture interrupt control functions
are updated to directly invoke the GIC driver functions.

This commit also adds support for the Cortex-R cores (Cortex-R4 and R5)
that allow interfacing to a custom external interrupt controller
(i.e. non-GIC) by introducing the `ARM_CUSTOM_INTERRUPT_CONTROLLER`
configuration that maps the architecture interrupt control functions to
the SoC layer interrupt control functions.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>