irqchip/dw-apb-ictl: Support building as module

The driver is now always built in. In some synaptics ARM64 SoCs it is used
as a second level interrupt controller hanging off the ARM GIC and is
therefore loadable during boot.

Enable it to be built as a module and handle built-in usage correctly, so
that it continues working on systems where it is the main interrupt
controller.

[ tglx: Massage changelog ]

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240614153449.2083-1-jszhang@kernel.org