MAINTAINERS: Add Purism mail alias as reviewer for their devkit's panel

Add a mail alias as reviewer for the rocktech jh057n00900 panel driver.
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/a392758c914d436ec5e449980f619bc906c3054a.1561542477.git.agx@sigxcpu.org

MAINTAINERS

@@ -5114,6 +5114,7 @@ F:	include/uapi/drm/r128_drm.h
 
 DRM DRIVER FOR ROCKTECH JH057N00900 PANELS
 M:	Guido Günther <agx@sigxcpu.org>
+R:	Purism Kernel Team <kernel@puri.sm>
 S:	Maintained
 F:	drivers/gpu/drm/panel/panel-rocktech-jh057n00900.c
 F:	Documentation/devicetree/bindings/display/panel/rocktech,jh057n00900.txt