irqchip: Add driver for Loongson-3 HyperTransport PIC controller

This controller appeared on Loongson-3 family of chips to receive
interrupts from PCH PIC.
It is a I8259 with optimized interrupt polling flow. We can poll
interrupt number from HT vector directly but still have to follow
standard I8259 routines to mask, unmask and EOI.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Co-developed-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>