irqchip/loongson-htvec: Support 8 groups of HT vectors

The original version can only used by old Loongson-3 which only use 4
groups of HT vectors. Now Loongson-3A R4 can use 8 groups, so improve
the driver to support all 8 groups.

Fixes: 818e915f ("irqchip: Add Loongson HyperTransport Vector support")
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Link: https://lore.kernel.org/r/1596099090-23516-5-git-send-email-chenhc@lemote.com