irqchip: Add Loongson HyperTransport Vector support

This controller appears on Loongson-3 chips for receiving interrupt
vectors from PCH's PIC and PCH's PCIe MSI interrupts.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200528152757.1028711-2-jiaxun.yang@flygoat.com