arch: arm: aarch64: Refactor interrupt interface

The current AArch64 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 ARMv8 cores (e.g. Cortex-A53)
that allow interfacing to a custom external interrupt controller
(i.e. non-GIC) by mapping the architecture interrupt control functions
to the SoC layer interrupt control functions when
`ARM_CUSTOM_INTERRUPT_CONTROLLER` configuration is enabled.

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