openpower: Add myself to maintainers

I look after pdbg, aspeed-lpc-ctrl, skiboot (pflash, gard) and
hostboot_settings in this repository.

Change-Id: I3f25e0dbd24621fd9cde254909f33a4528bad949
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/MAINTAINERS b/MAINTAINERS
index bfe81de..d95ebb6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -43,3 +43,4 @@
 -------------------------
 
 M:  Brad Bishop <bradleyb@fuzziesquirrel.com> <radsquirrel!>
+M:  Joel Stanley <joel@jms.id.au> <shenki!>