Maintainers.txt: add reviewer to HiSilicon
Add myself to Maintainers.txt as a Hisilicon reviewer.
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Signed-off-by: Wenyi Xie <xiewenyi2@huawei.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>