irqchip: Add Loongson PCH PIC controller

This controller appears on Loongson LS7A family of PCH to transform
interrupts from devices into HyperTransport vectorized interrrupts
and send them to procrssor's HT vector controller.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200528152757.1028711-4-jiaxun.yang@flygoat.com