MAINTAINERS: Add myself as reviewer for the hwspinlock subsystem

I spend some time to do some optimization for the hwspinlock subsystem
in the nearest past, and I am willing to be a reviewer.

Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Link: https://lore.kernel.org/r/0a6d0c22100da5196f00dfe0eb431e5e6d6d8c65.1575454108.git.baolin.wang7@gmail.com


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>