irqchip/loongson-pch-pic: 7a1000 int_clear reg must use 64bit write.

LoongArch inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7BUJN



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Signed-off-by: Chong Qiao <qiaochong@loongson.cn>
Signed-off-by: Hongchen Zhang <zhanghongchen@loongson.cn>
(cherry picked from commit 3b5bcd2a)