arch: arm: cortex-m: support custom interrupt controllers

This change adds support for the CONFIG_ARM_CUSTOM_INTERRUPT_CONTROLLER
option on Cortex-M platforms. While all Cortex-M platforms have a
NVIC controller some custom SoCs may have additional IRQ controllers
or custom handling. This change allows those SoCs to modify this
bahaviour without having to place platform specific logic inside
applications or drivers.

Signed-off-by: Corey Wharton <xodus7@cwharton.com>