Dtoverlay:Add waveshare 4inch dsi lcd (C) parameters to the README

Signed-off-by: Eng33 <eng33@waveshare.net>

Author: Eng33

arch/arm/boot/dts/overlays/README

@@ -5011,6 +5011,7 @@ Load:   dtoverlay=vc4-kms-dsi-waveshare-panel,<param>=<val>
 Params: 2_8_inch                2.8" 480x640
         3_4_inch                3.4" 800x800 round
         4_0_inch                4.0" 480x800
+        4_0_inchC               4.0" 720x720
         7_0_inchC               7.0" C 1024x600
         7_9_inch                7.9" 400x1280
         8_0_inch                8.0" 1280x800