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>

doc/git-mailrc

@@ -106,6 +106,7 @@ alias sh             superh
 alias x86            uboot, sjg, gruss
 
 # Subsystem aliases
+alias dm             uboot, sjg
 alias cfi            uboot, stroese
 alias dfu            uboot, lukma
 alias kerneldoc      uboot, marex