git-mailrc: Add myself as dm maintainer

Add a subsystem entry for dm with myself as maintainer.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/git-mailrc b/doc/git-mailrc
index 947f669..70405f2 100644
--- a/doc/git-mailrc
+++ b/doc/git-mailrc
@@ -106,6 +106,7 @@
 alias x86            uboot, sjg, gruss
 
 # Subsystem aliases
+alias dm             uboot, sjg
 alias cfi            uboot, stroese
 alias dfu            uboot, lukma
 alias kerneldoc      uboot, marex