ARM: shmobile: mackerel: Remove DT binding documentation

Remove the DT compatible string documentation for the
no longer supported Mackerel board.
Signed-off-by: NMagnus Damm <damm+renesas@opensource.se>
Acked-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>

Documentation/devicetree/bindings/arm/shmobile.txt

@@ -61,8 +61,6 @@ Boards:
     compatible = "renesas,kzm9g-reference", "renesas,sh73a0"
   - Lager (RTP0RC7790SEB00010S)
     compatible = "renesas,lager", "renesas,r8a7790"
-  - Mackerel (R0P7372LC0016RL, AP4 EVM 2nd)
-    compatible = "renesas,mackerel"
   - Marzen
     compatible = "renesas,marzen", "renesas,r8a7779"