irqchip: Adjust Kconfig for Loongson

HTVEC will be shared by both MIPS-based and LoongArch-based Loongson
processors (not only Loongson-3), so we adjust its description. HTPIC is
only used by MIPS-based Loongson, so we add a MIPS dependency.

Acked-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>