mailbox: hisilicon: Use the correct HiSilicon copyright

s/Hisilicon/HiSilicon/.
It should use capital S, according to
https://www.hisilicon.com/en

.

Signed-off-by: Hao Fang <fanghao11@huawei.com>
Reviewed-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>