irqchip: Add support for Realtek RTL838x/RTL839x interrupt controller

This is a standard IRQ driver with only status and mask registers.

The mapping from SoC interrupts (18-31) to MIPS core interrupts is
done via an interrupt-map in device tree.

Signed-off-by: Bert Vermeulen <bert@biot.com>
Signed-off-by: Birger Koblitz <mail@birger-koblitz.de>
Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210122204224.509124-3-bert@biot.com