dt-bindings: interrupt-controller: Update Loongson HTVEC description

Loongson HTVEC support 8 parents interrupts in maximum, so update the
maxItems description.

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/1596099090-23516-2-git-send-email-chenhc@lemote.com