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

MAINTAINERS

@@ -7289,6 +7289,7 @@ F:	drivers/hwtracing/
 HARDWARE SPINLOCK CORE
 M:	Ohad Ben-Cohen <ohad@wizery.com>
 M:	Bjorn Andersson <bjorn.andersson@linaro.org>
+R:	Baolin Wang <baolin.wang7@gmail.com>
 L:	linux-remoteproc@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/andersson/remoteproc.git hwspinlock-next